breaking: throw on missing dynamic import when import() is seen in systemjs (#12700)

packages/babel-plugin-proposal-dynamic-import/test/fixtures/systemjs/missing-plugin-babel-7/input.mjs

@@ -0,0 +1,3 @@
+import "a";
+
+import("b");

packages/babel-plugin-proposal-dynamic-import/test/fixtures/systemjs/missing-plugin-babel-7/options.json

@@ -0,0 +1,8 @@
+{
+  "validateLogs": true,
+  "plugins": [
+    "transform-modules-systemjs",
+    "external-helpers"
+  ],
+  "BABEL_8_BREAKING": false
+}

packages/babel-plugin-proposal-dynamic-import/test/fixtures/systemjs/missing-plugin-babel-7/output.mjs

@@ -4,7 +4,6 @@ System.register(["a"], function (_export, _context) {
   return {
     setters: [function (_a) {}],
     execute: function () {
-      // TODO: This should throw in Babel 8
       _context.import("b");
     }
   };

packages/babel-plugin-proposal-dynamic-import/test/fixtures/systemjs/missing-plugin/input.mjs

@@ -1,5 +1,3 @@
-// TODO: This should throw in Babel 8
-
 import "a";
 
 import("b");

packages/babel-plugin-proposal-dynamic-import/test/fixtures/systemjs/missing-plugin/options.json

@@ -1,7 +1,8 @@
 {
-  "validateLogs": true,
   "plugins": [
     "transform-modules-systemjs",
     "external-helpers"
-  ]
+  ],
+  "BABEL_8_BREAKING": true,
+  "throws": "ERROR: Dynamic import() transformation must be enabled using the\n       @babel/plugin-proposal-dynamic-import plugin. Babel 8\n       no longer transforms import() without using that plugin."
 }

packages/babel-plugin-transform-modules-systemjs/src/index.js

@@ -28,6 +28,12 @@ WARNING: Dynamic import() transformation must be enabled using the
          no longer transform import() without using that plugin.
 `;
 
+const MISSING_PLUGIN_ERROR = `\
+ERROR: Dynamic import() transformation must be enabled using the
+       @babel/plugin-proposal-dynamic-import plugin. Babel 8
+       no longer transforms import() without using that plugin.
+`;
+
 //todo: use getExportSpecifierName in `helper-module-transforms` when this library is refactored to NodePath usage.
 
 export function getExportSpecifierName(
@@ -226,8 +232,12 @@ export default declare((api, options) => {
       CallExpression(path, state: PluginState) {
         if (t.isImport(path.node.callee)) {
           if (!this.file.has("@babel/plugin-proposal-dynamic-import")) {
+            if (process.env.BABEL_8_BREAKING) {
+              throw new Error(MISSING_PLUGIN_ERROR);
+            } else {
               console.warn(MISSING_PLUGIN_WARNING);
             }
+          }
 
           path.replaceWith(
             t.callExpression(