Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/canvas_test.dart

+library CanvasTest;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/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);
+    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));
+  });
+
+  test('toDataUrl', () {
+    var canvas = new CanvasElement(width: 100, height: 100);
+    var context = canvas.context2D;
+    context.fillStyle = 'red';
+    context.fill();
+
+    var url = canvas.toDataUrl();
+
+    var img = new ImageElement();
+    img.onLoad.listen(expectAsync((_) {
+      expect(img.complete, true);
+    }));
+    img.onError.listen((_) {
+      fail('URL failed to load.');
+    });
+    img.src = url;
+  });
+}
+
+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]));
+  }
+}