Add Path#isInStrictMode (#7712)

* Add Path#isInStrictMode

* Fix undefined directives

* Explicitly return for arrow expressions

packages/babel-traverse/src/path/introspection.js

@@ -439,3 +439,33 @@ export function isConstantExpression() {
 
   return false;
 }
+
+export function isInStrictMode() {
+  const start = this.isProgram() ? this : this.parentPath;
+
+  const strictParent = start.find(path => {
+    if (path.isProgram({ sourceType: "module" })) return true;
+
+    if (path.isClass()) return true;
+
+    if (!path.isProgram() && !path.isFunction()) return false;
+
+    if (
+      path.isArrowFunctionExpression() &&
+      !path.get("body").isBlockStatement()
+    ) {
+      return false;
+    }
+
+    let { node } = path;
+    if (path.isFunction()) node = node.body;
+
+    for (const directive of node.directives) {
+      if (directive.value.value === "use strict") {
+        return true;
+      }
+    }
+  });
+
+  return !!strictParent;
+}