Some HTML API cleanup

lib/dom/templates/html/impl/impl_CanvasElement.darttemplate

Index: lib/dom/templates/html/impl/impl_CanvasElement.darttemplate
diff --git a/lib/dom/templates/dom/native/dart_implementation.darttemplate b/lib/dom/templates/html/impl/impl_CanvasElement.darttemplate
similarity index 67%
copy from lib/dom/templates/dom/native/dart_implementation.darttemplate
copy to lib/dom/templates/html/impl/impl_CanvasElement.darttemplate
index 885414fa4e9c02d40a38aa12d0718ccc0d47f85e..61ce64c22a825bf8a11f9d68069653852b42ac4c 100644
--- a/lib/dom/templates/dom/native/dart_implementation.darttemplate
+++ b/lib/dom/templates/html/impl/impl_CanvasElement.darttemplate
@@ -2,8 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// WARNING: Do not edit - generated code.
-
-class $CLASSNAME$EXTENDS$IMPLEMENTS {
+class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
+
+  CanvasRenderingContext2D get context2d() => getContext('2d');
 }