diff --git a/packages/babel-core/test/api.js b/packages/babel-core/test/api.js
index 99259ce4c1..2be88b495a 100644
--- a/packages/babel-core/test/api.js
+++ b/packages/babel-core/test/api.js
@@ -285,16 +285,7 @@ describe("api", function() {
 
     assert.equal(aliasBaseType, "NumberTypeAnnotation");
 
-    assert.deepEqual(
-      [
-        '"use strict";',
-        "",
-        "var x = function x(y) {",
-        "  return y;",
-        "};",
-      ].join("\n"),
-      result.code,
-    );
+    assert.deepEqual(result.code, "var x = function x(y) {\n  return y;\n};");
 
     // 2. passPerPreset: false
 
@@ -304,16 +295,7 @@ describe("api", function() {
 
     assert.equal(aliasBaseType, null);
 
-    assert.deepEqual(
-      [
-        '"use strict";',
-        "",
-        "var x = function x(y) {",
-        "  return y;",
-        "};",
-      ].join("\n"),
-      result.code,
-    );
+    assert.deepEqual(result.code, "var x = function x(y) {\n  return y;\n};");
   });
 
   it("complex plugin and preset ordering", function() {
diff --git a/packages/babel-core/test/fixtures/transformation/misc/regression-4855/expected.js b/packages/babel-core/test/fixtures/transformation/misc/regression-4855/expected.js
index 0504b31e42..3b48a02697 100644
--- a/packages/babel-core/test/fixtures/transformation/misc/regression-4855/expected.js
+++ b/packages/babel-core/test/fixtures/transformation/misc/regression-4855/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var _values = values;
 var _fieldName = fieldName;
 value = _values[_fieldName];
diff --git a/packages/babel-helper-module-transforms/src/index.js b/packages/babel-helper-module-transforms/src/index.js
index 6d40af3f96..f868c01fc3 100644
--- a/packages/babel-helper-module-transforms/src/index.js
+++ b/packages/babel-helper-module-transforms/src/index.js
@@ -1,3 +1,4 @@
+import assert from "assert";
 import * as t from "babel-types";
 import template from "babel-template";
 import chunk from "lodash/chunk";
@@ -11,6 +12,31 @@ import normalizeAndLoadModuleMetadata, {
 
 export { hasExports, isSideEffectImport };
 
+export function isModule(path: NodePath, requireUnambiguous: boolean = false) {
+  const { sourceType } = path.node;
+  if (sourceType !== "module" && sourceType !== "script") {
+    throw path.buildCodeFrameError(
+      `Unknown sourceType "${sourceType}", cannot transform.`,
+    );
+  }
+
+  const filename = path.hub.file.opts.filename;
+  if (/\.mjs$/.test(filename)) {
+    requireUnambiguous = false;
+  }
+
+  return (
+    path.node.sourceType === "module" &&
+    (!requireUnambiguous || isUnambiguousModule(path))
+  );
+}
+
+// This approach is not ideal. It is here to preserve compatibility for now,
+// but really this should just return true or be deleted.
+function isUnambiguousModule(path) {
+  return path.get("body").some(p => p.isModuleDeclaration());
+}
+
 /**
  * Perform all of the generic ES6 module rewriting needed to handle initial
  * module processing. This function will rewrite the majority of the given
@@ -21,6 +47,9 @@ export function rewriteModuleStatementsAndPrepareHeader(
   path: NodePath,
   { exportName, strict, allowTopLevelThis, strictMode, loose, noInterop },
 ) {
+  assert(isModule(path), "Cannot process module statements in a script");
+  path.node.sourceType = "script";
+
   const meta = normalizeAndLoadModuleMetadata(path, exportName, {
     noInterop,
   });
diff --git a/packages/babel-plugin-transform-class-properties/test/fixtures/general/foobar/expected.js b/packages/babel-plugin-transform-class-properties/test/fixtures/general/foobar/expected.js
index 3532914e26..2865c35414 100644
--- a/packages/babel-plugin-transform-class-properties/test/fixtures/general/foobar/expected.js
+++ b/packages/babel-plugin-transform-class-properties/test/fixtures/general/foobar/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var Child =
 /*#__PURE__*/
 function (_Parent) {
diff --git a/packages/babel-plugin-transform-class-properties/test/fixtures/loose/foobar/expected.js b/packages/babel-plugin-transform-class-properties/test/fixtures/loose/foobar/expected.js
index cf6a9bc02c..f43f546b7c 100644
--- a/packages/babel-plugin-transform-class-properties/test/fixtures/loose/foobar/expected.js
+++ b/packages/babel-plugin-transform-class-properties/test/fixtures/loose/foobar/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var Child =
 /*#__PURE__*/
 function (_Parent) {
diff --git a/packages/babel-plugin-transform-class-properties/test/fixtures/regression/6154/expected.js b/packages/babel-plugin-transform-class-properties/test/fixtures/regression/6154/expected.js
index 01805e5c56..c027b23cbc 100644
--- a/packages/babel-plugin-transform-class-properties/test/fixtures/regression/6154/expected.js
+++ b/packages/babel-plugin-transform-class-properties/test/fixtures/regression/6154/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 function _typeof(obj) { if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") { _typeof = function _typeof(obj) { return typeof obj; }; } else { _typeof = function _typeof(obj) { return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj; }; } return _typeof(obj); }
 
 function _instanceof(left, right) { if (right != null && typeof Symbol !== "undefined" && right[Symbol.hasInstance]) { return right[Symbol.hasInstance](left); } else { return left instanceof right; } }
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/expected.js
index 2fe6ea970f..0abfa635e3 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var A =
 /*#__PURE__*/
 function (_B) {
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2494/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2494/expected.js
index 86a3c654b2..7ee2040344 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2494/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2494/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var x = {
   Foo:
   /*#__PURE__*/
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2997/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2997/expected.js
index b46893d0e6..8ecfecbaaa 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2997/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T2997/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var A = function A() {
   babelHelpers.classCallCheck(this, A);
 };
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/expected.js
index 96e00e6922..08cac4b578 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6712/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var A =
 /*#__PURE__*/
 function () {
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6755/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6755/expected.js
index b5fa2e3253..6b2fd1e691 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6755/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T6755/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var Example =
 /*#__PURE__*/
 function () {
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7010/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7010/expected.js
index 3d7a460b5a..4f8dfbdae2 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7010/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7010/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var Foo =
 /*#__PURE__*/
 function () {
diff --git a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7537/expected.js b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7537/expected.js
index ae764cf723..1a7251e480 100644
--- a/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7537/expected.js
+++ b/packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/T7537/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 function _typeof(obj) { if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") { _typeof = function _typeof(obj) { return typeof obj; }; } else { _typeof = function _typeof(obj) { return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj; }; } return _typeof(obj); }
 
 function _possibleConstructorReturn(self, call) { if (call && (_typeof(call) === "object" || typeof call === "function")) { return call; } if (!self) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return self; }
diff --git a/packages/babel-plugin-transform-es2015-modules-amd/src/index.js b/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
index 4c2fce1bf6..b549ac4204 100644
--- a/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-amd/src/index.js
@@ -1,5 +1,6 @@
 import template from "babel-template";
 import {
+  isModule,
   rewriteModuleStatementsAndPrepareHeader,
   hasExports,
   isSideEffectImport,
@@ -18,6 +19,8 @@ export default function({ types: t }) {
     visitor: {
       Program: {
         exit(path, state) {
+          if (!isModule(path)) return;
+
           const {
             loose,
             allowTopLevelThis,
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js b/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
index b52cfd0d1e..b7119c8774 100644
--- a/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js
@@ -1,4 +1,5 @@
 import {
+  isModule,
   rewriteModuleStatementsAndPrepareHeader,
   isSideEffectImport,
   buildNamespaceInitStatements,
@@ -11,6 +12,10 @@ export default function({ types: t }) {
     visitor: {
       Program: {
         exit(path, state) {
+          // For now this requires unambiguous rather that just sourceType
+          // because Babel currently parses all files as sourceType:module.
+          if (!isModule(path, true /* requireUnambiguous */)) return;
+
           const {
             loose,
             allowTopLevelThis,
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-arrow-function/actual.js b/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-arrow-function/actual.mjs
similarity index 100%
rename from packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-arrow-function/actual.js
rename to packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-arrow-function/actual.mjs
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-call/actual.js b/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-call/actual.mjs
similarity index 100%
rename from packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-call/actual.js
rename to packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-call/actual.mjs
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-declaration/actual.js b/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-declaration/actual.mjs
similarity index 100%
rename from packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-declaration/actual.js
rename to packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-declaration/actual.mjs
diff --git a/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-reference/actual.js b/packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-reference/actual.mjs
similarity index 100%
rename from packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-reference/actual.js
rename to packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/misc/undefined-this-root-reference/actual.mjs
diff --git a/packages/babel-plugin-transform-es2015-modules-umd/src/index.js b/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
index 1cf4fa9f05..c984cb53e7 100644
--- a/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
+++ b/packages/babel-plugin-transform-es2015-modules-umd/src/index.js
@@ -1,6 +1,7 @@
 import { basename, extname } from "path";
 import template from "babel-template";
 import {
+  isModule,
   rewriteModuleStatementsAndPrepareHeader,
   hasExports,
   isSideEffectImport,
@@ -107,6 +108,8 @@ export default function({ types: t }) {
     visitor: {
       Program: {
         exit(path, state) {
+          if (!isModule(path)) return;
+
           const {
             globals,
             exactGlobals,
diff --git a/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-loose-return-type-annotation/expected.js b/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-loose-return-type-annotation/expected.js
index 16ca53f948..d39c634e40 100644
--- a/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-loose-return-type-annotation/expected.js
+++ b/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-loose-return-type-annotation/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 // @flow
 var C =
 /*#__PURE__*/
diff --git a/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-return-type-annotation/expected.js b/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-return-type-annotation/expected.js
index 68d6c4494b..cca8f738c3 100644
--- a/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-return-type-annotation/expected.js
+++ b/packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/transformed-class-method-return-type-annotation/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 // @flow
 var C =
 /*#__PURE__*/
diff --git a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/expected.js b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/expected.js
index 42a2b16879..f5f9fc21cb 100644
--- a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/expected.js
+++ b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-loose-return-type-annotation/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var C =
 /*#__PURE__*/
 function () {
diff --git a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-return-type-annotation/expected.js b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-return-type-annotation/expected.js
index 8613c02f7e..8665e0ced3 100644
--- a/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-return-type-annotation/expected.js
+++ b/packages/babel-plugin-transform-flow-strip-types/test/fixtures/regression/transformed-class-method-return-type-annotation/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 var C =
 /*#__PURE__*/
 function () {
diff --git a/packages/babel-plugin-transform-regenerator/test/fixtures/regression/4219/expected.js b/packages/babel-plugin-transform-regenerator/test/fixtures/regression/4219/expected.js
index ffaa4b9a4e..babd815f2d 100644
--- a/packages/babel-plugin-transform-regenerator/test/fixtures/regression/4219/expected.js
+++ b/packages/babel-plugin-transform-regenerator/test/fixtures/regression/4219/expected.js
@@ -1,5 +1,3 @@
-"use strict";
-
 function test(fn) {
   return function _callee() {
     var _args = arguments;
diff --git a/packages/babel-preset-es2015/test/fixtures/preset-options/spec/expected.js b/packages/babel-preset-es2015/test/fixtures/preset-options/spec/expected.js
index e4b0c4adb6..889d4b054a 100644
--- a/packages/babel-preset-es2015/test/fixtures/preset-options/spec/expected.js
+++ b/packages/babel-preset-es2015/test/fixtures/preset-options/spec/expected.js
@@ -1,11 +1,9 @@
-"use strict";
-
-var _this = void 0;
+var _this = this;
 
 "1".concat(a);
 (function () {
   babelHelpers.newArrowCheck(this, _this);
-}).bind(void 0);
+}).bind(this);
 
 function a() {
   var _this2 = this;
diff --git a/packages/babel-standalone/test/babel.js b/packages/babel-standalone/test/babel.js
index 6dd00e9695..75f3d220dd 100644
--- a/packages/babel-standalone/test/babel.js
+++ b/packages/babel-standalone/test/babel.js
@@ -18,7 +18,7 @@ describe("babel-standalone", () => {
     const output = Babel.transform("class A {}", {
       presets: ["es2015-loose"],
     }).code;
-    assert.equal(output, '"use strict";\n\nvar A = function A() {};');
+    assert.equal(output, "var A = function A() {};");
   });
   it("handles the typescript preset", () => {
     const output = Babel.transform("var a: string;", {
@@ -56,7 +56,7 @@ describe("babel-standalone", () => {
     };
     const output = Babel.transformFromAst(ast, "42", { presets: ["es2015"] })
       .code;
-    assert.equal(output, '"use strict";\n' + "\n" + "42;");
+    assert.equal(output, "42;");
   });
 
   it("handles the react preset", () => {