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

tests/html/url_test.dart

Index: tests/html/url_test.dart
diff --git a/tests/html/url_test.dart b/tests/html/url_test.dart
index 2b3260bdb3b9ae40a5bab87a41fa7fd296c06622..538465a07c79e9bc88aaef6bdbf585a17b9153e8 100644
--- a/tests/html/url_test.dart
+++ b/tests/html/url_test.dart
@@ -18,7 +18,8 @@ main() {
 
     var context = canvas.context2D;
     context.fillStyle = 'red';
-    context.fillRect(0, 0, canvas.width, canvas.height);
+    context.fillRect(0.0, 0.0,
+        canvas.width.toDouble(), canvas.height.toDouble());
 
     var dataUri = canvas.toDataUrl('image/png');
     var byteString = window.atob(dataUri.split(',')[1]);