fixes #157, renaming local library identifiers if needed.

test/codegen/expect/names.js

Index: test/codegen/expect/names.js
diff --git a/test/codegen/expect/names.js b/test/codegen/expect/names.js
index 190c6fb48df0d845a42ae630e33a20c4ae85d403..40710ab76e35c13c0694914ba7981e5f54df0c58 100644
--- a/test/codegen/expect/names.js
+++ b/test/codegen/expect/names.js
@@ -1,5 +1,6 @@
-var names;
-(function(exports) {
+var names = dart.defineLibrary(names, {});
+var core = dart.import(core);
+(function(exports, core) {
   'use strict';
   exports.exports = 42;
   let _foo$ = Symbol('_foo');
@@ -47,4 +48,4 @@ var names;
   exports.Frame = Frame;
   exports.Frame2 = Frame2;
   exports.main = main;
-})(names || (names = {}));
+})(names, core);