Tolerate undeclared exports until @babel/parser@7.5.0 is released.

babel/babel#9864

This hack narrowly targets @babel/parser@7.4.x by tricking
Array.from(this.scope.undefinedExports) into returning an empty array,
thereby simulating the effect of enabling the allowUndeclaredExports
option (which will become available in @babel/parser@7.5.0).

lib/parsers/babel.js

@@ -1,18 +1,13 @@
 "use strict";
 
-// Prefer the new @babel/parser package, but fall back to babylon if
-// that's what's available.
-const babelParser = function () {
-  try {
-    return require("@babel/parser");
-  } catch (e) {
-    return require("babylon");
-  }
-}();
+const babelParser = require("@babel/parser");
+const babelParserVersion = require("@babel/parser/package.json").version;
 
 exports.options = {
   allowImportExportEverywhere: true,
   allowReturnOutsideFunction: true,
+  // This won't work until @babel/parser@7.5.0 is released.
+  allowUndeclaredExports: true,
   plugins: [
     "*", "flow", "jsx",
     // The "*" glob no longer seems to include the following plugins:
@@ -55,4 +50,24 @@ function parse(code) {
   return babelParser.parse(code, exports.options);
 }
 
-exports.parse = parse;
+function tolerantParse(code) {
+  const arrayFrom = Array.from;
+  // There is only one use of Array.from in the @babel/parser@7.4.x code,
+  // Array.from(this.scope.undefinedExports), which determines whether the
+  // parser complains prematurely about exporting identifiers that were
+  // not declared in the current module scope. By returning an empty array
+  // when the source argument is a Map, we can effectively disable that
+  // error behavior, until https://github.com/babel/babel/pull/9864 is
+  // released in @babel/parser@7.5.0.
+  Array.from = function (source) {
+    return source instanceof Map ? [] :
+      arrayFrom.apply(this, arguments);
+  };
+  try {
+    return parse(code);
+  } finally {
+    Array.from = arrayFrom;
+  }
+}
+
+exports.parse = babelParserVersion.startsWith("7.4.") ? tolerantParse : parse;

test/export-tests.js

@@ -317,4 +317,9 @@ describe("export declarations", () => {
       "from-ordering-c.js",
     ]);
   });
+
+  it("should be tolerant of exporting undeclared identifiers", () => {
+    import { GlobalArray } from "./undeclared-export.js";
+    assert.strictEqual(GlobalArray, Array);
+  });
 });

test/undeclared-export.js

@@ -0,0 +1 @@
+export { Array as GlobalArray }