fixes #157, renaming local library identifiers if needed.

test/codegen/expect/sunflower/dom.js

-var dom;
-(function(exports) {
+var dom = dart.defineLibrary(dom, window);
+var core = dart.import(core);
+(function(exports, core) {
   'use strict';
   class JsName extends core.Object {
     JsName(opts) {
       let name = opts && 'name' in opts ? opts.name : null;
       this.name = name;
     }
   }
   class Overload extends core.Object {
     Overload() {
     }
@@ skipping 21 matching lines @@
   exports.JsName = JsName;
   exports.Overload = Overload;
   exports.overload = overload;
   exports.EventListener = EventListener;
   exports.Event = Event;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
-})(dom || (dom = window));
+})(dom, core);