Exposing DOM float & double types as double rather than num.

tests/html/canvas_test.dart

 library CanvasTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   CanvasElement canvas;
   CanvasRenderingContext2D context;
   int width = 100;
   int height = 100;
 
   canvas = new CanvasElement(width:width, height:height);
   document.body.append(canvas);
 
   context = canvas.context2D;
 
   useHtmlConfiguration();
   test('CreateImageData', () {
-    ImageData image = context.createImageData(canvas.width,
-                                              canvas.height);
+    ImageData image = context.createImageData(canvas.width.toDouble(),
+                                              canvas.height.toDouble());
     List<int> data = image.data;
 
     expect(data, hasLength(40000));
     checkPixel(data, 0, [0, 0, 0, 0]);
     checkPixel(data, width * height - 1, [0, 0, 0, 0]);
 
     data[100] = 200;
     expect(data[100], equals(200));
   });
 
@@ skipping 18 matching lines @@
   });
 }
 
 void checkPixel(List<int> data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(data[offset + i], equals(rgba[i]));
   }
 }