fix sunflower -- dom types were not resolving

test/codegen/expect/sunflower/dom.js

 var dom;
 (function(exports) {
   '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() {
     }
   }
   let overload = new Overload();
   let EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));
   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.Document = Document;
+  exports.Element = Element;
   exports.EventListener = EventListener;
   exports.Event = Event;
+  exports.InputElement = InputElement;

[Jacob, 2015/04/13 19:27:14]

all these lines except for ones that actually have a rename are wasteful

exports starts out as window which already has all these members on it.
options:
restrict exports to only contain the explicitly exposed classes (possibly better
for debugging but more fragile and slower) 
keep having 
dom = window
and only add exports that have an explicit Dart rename.

[Jennifer Messerly, 2015/04/13 20:02:10]

hmmm. That's a good point, I forgot that we make "window" the module global for
"dom". That's probably not going to work in ES6 ... maybe we could re-export all
names:

    export { * } from 'window';

or have smarts when we generate imports:

    import { Document } from 'window';
    import { InputElement } from 'dart/dom';

or even smarter:

    import { Document, InputElement as HTMLInputElement } from 'window';

+  exports.CanvasElement = CanvasElement;
   exports.RenderingContext = RenderingContext;
+  exports.CanvasRenderingContext2D = CanvasRenderingContext2D;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
+  exports.CanvasGradient = CanvasGradient;
+  exports.CanvasPattern = CanvasPattern;
+  exports.TextMetrics = TextMetrics;
+  exports.ImageData = ImageData;
 })(dom || (dom = window));