More precise native type for context2D

tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate

Index: tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
index 82bf5a766032ca6d9753fc0deef37a4552ec3a57..ef01381ece5b7505308c93fb7ff772ab91e62765 100644
--- a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
@@ -7,7 +7,12 @@ part of $LIBRARYNAME;
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS implements CanvasImageSource$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /** An API for drawing on this canvas. */
+$if DART2JS
+  CanvasRenderingContext2D get context2D =>
+      JS('Null|CanvasRenderingContext2D', '#.getContext(#)', this, '2d');

[sra1, 2013/04/09 04:14:24]

Since this avoids calling the Dart this.getContext, there is no code for parsing
the bag of options.
That accounts of 1k of the saving.

The other 2k comes from avoiding intereceptors for the drawing methods since the
CanvasRenderingContext2D type is propagated instead of CanvasRenderingContext.

+$else
   CanvasRenderingContext2D get context2D => getContext('2d');
+$endif
 
   @deprecated
   CanvasRenderingContext2D get context2d => this.context2D;