fixes #157, renaming local library identifiers if needed.

lib/runtime/dart/core.js

Index: lib/runtime/dart/core.js
diff --git a/lib/runtime/dart/core.js b/lib/runtime/dart/core.js
index a3a7973d9cd5d50e84cca4b58776421e75e66290..f74ea9c540a92c0b57ac833ce2e8e19afc3a0de8 100644
--- a/lib/runtime/dart/core.js
+++ b/lib/runtime/dart/core.js
@@ -1,5 +1,5 @@
-var core;
-(function(exports) {
+var core, _js_helper, _internal, collection, _interceptors, convert;
+(function(exports, _js_helper, _internal, collection, _interceptors, convert) {
   'use strict';
   class Object {
     constructor() {
@@ -3486,4 +3486,4 @@ var core;
   exports.Type = Type;
   exports.Uri = Uri;
   exports.SupportJsExtensionMethods = SupportJsExtensionMethods;
-})(core || (core = {}));
+})(core || (core = {}), _js_helper || (_js_helper = {}), _internal || (_internal = {}), collection || (collection = {}), _interceptors || (_interceptors = {}), convert || (convert = {}));