Add a native subtype of function that uses the native C++ class hierarchy to check if an element

Source/bindings/dart/custom/DartCanvasRenderingContext2DCustom.cpp

Index: Source/bindings/dart/custom/DartCanvasRenderingContext2DCustom.cpp
===================================================================
--- Source/bindings/dart/custom/DartCanvasRenderingContext2DCustom.cpp	(revision 1437)
+++ Source/bindings/dart/custom/DartCanvasRenderingContext2DCustom.cpp	(working copy)
@@ -70,11 +70,10 @@
 static PassRefPtr<CanvasStyle> toCanvasStyle(Dart_Handle value)
 {
     Dart_Handle exception = 0;
-    DartDOMData* domData = DartDOMData::current();
-    if (DartDOMWrapper::instanceOf<DartCanvasGradient>(domData, value))
+    if (DartDOMWrapper::subtypeOf(value, DartCanvasGradient::dartClassId))
         return CanvasStyle::createFromGradient(DartCanvasGradient::toNative(value, exception));
 
-    if (DartDOMWrapper::instanceOf<DartCanvasPattern>(domData, value))
+    if (DartDOMWrapper::subtypeOf(value, DartCanvasPattern::dartClassId))
         return CanvasStyle::createFromPattern(DartCanvasPattern::toNative(value, exception));
 
     return 0;