build: mjs files import other files using full paths with exten… (#2379)

Motivation #2277

.babelrc

@@ -19,7 +19,8 @@
           "presets": [["@babel/preset-env", { "modules": "commonjs" }]]
         },
         "mjs": {
-          "presets": [["@babel/preset-env", { "modules": false }]]
+          "presets": [["@babel/preset-env", { "modules": false }]],
+          "plugins": ['./resources/add-extension-to-import-paths']
         }
       }
     },

resources/add-extension-to-import-paths.js

@@ -0,0 +1,43 @@
+// @noflow
+
+'use strict';
+
+/**
+ * Adds extension to all paths imported inside MJS files
+ *
+ * Transforms:
+ *
+ *  import { foo } from './bar';
+ *  export { foo } from './bar';
+ *
+ * to:
+ *
+ *  import { foo } from './bar.mjs';
+ *  export { foo } from './bar.mjs';
+ *
+ */
+module.exports = function addExtensionToImportPaths(context) {
+  const { types } = context;
+
+  return {
+    visitor: {
+      ImportDeclaration: replaceImportPath,
+      ExportNamedDeclaration: replaceImportPath,
+    },
+  };
+
+  function replaceImportPath(path) {
+    // bail if the declaration doesn't have a source, e.g. "export { foo };"
+    if (!path.node.source) {
+      return;
+    }
+
+    const source = path.node.source.value;
+    if (source.startsWith('./') || source.startsWith('../')) {
+      if (!source.endsWith('.mjs')) {
+        const newSourceNode = types.stringLiteral(source + '.mjs');
+        path.get('source').replaceWith(newSourceNode);
+      }
+    }
+  }
+};