fixes #157, renaming local library identifiers if needed.

lib/runtime/dart/core.js

-var core;
-(function(exports) {
+var core = dart.defineLibrary(core, {});
+var _js_helper = dart.lazyImport(_js_helper);
+var _internal = dart.lazyImport(_internal);
+var collection = dart.lazyImport(collection);
+var _interceptors = dart.lazyImport(_interceptors);
+var convert = dart.lazyImport(convert);
+(function(exports, _js_helper, _internal, collection, _interceptors, convert) {
   'use strict';
   class Object {
     constructor() {
       let name = this.constructor.name;
       let init = this[name];
       let result = void 0;
       if (init)
         result = init.apply(this, arguments);
       return result === void 0 ? this : result;
     }
@@ skipping 3466 matching lines @@
   exports.StackTrace = StackTrace;
   exports.Stopwatch = Stopwatch;
   exports.String = String;
   exports.RuneIterator = RuneIterator;
   exports.StringBuffer = StringBuffer;
   exports.StringSink = StringSink;
   exports.Symbol = Symbol;
   exports.Type = Type;
   exports.Uri = Uri;
   exports.SupportJsExtensionMethods = SupportJsExtensionMethods;
-})(core || (core = {}));
+})(core, _js_helper, _internal, collection, _interceptors, convert);