fixes #173, ability to use native JS indexers

test/codegen/expect/sunflower/dom.js

 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;
     }
   }
   dart.setSignature(JsName, {
     constructors: () => ({JsName: [JsName, [], {name: core.String}]})
   });
   class Overload extends core.Object {
     Overload() {
     }
   }
   dart.setSignature(Overload, {
     constructors: () => ({Overload: [Overload, []]})
   });
   let overload = dart.const(new Overload());
   let EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));

[vsm, 2015/06/05 19:22:05]

Event is gone, but it's referred to right here still.

[Jennifer Messerly, 2015/06/05 19:24:37]

just chatted ... I think what happened here is Event is now @JsName, so we don't
generate it, on the theory that it's already a global.

-  class Event extends core.Object {}
   let InputElement = HTMLInputElement;
   let CanvasElement = HTMLCanvasElement;
   class RenderingContext extends core.Object {}
   class CanvasDrawingStyles extends core.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
       this.lineCap = null;
       this.lineJoin = null;
       this.miterLimit = null;
       this.lineDashOffset = null;
       this.font = null;
       this.textAlign = null;
       this.textBaseline = null;
     }
   }
   class CanvasPathMethods extends core.Object {}
   // Exports:
   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, core);