Pixels class prototype and test (not to be committed).

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
diff --git a/runtime/vm/raw_object.cc b/runtime/vm/raw_object.cc
index 73070ac2451388bbe710e2a7e5e827bd1c537922..cd5cd8c79f3c58b914aaa7fb7d5a971770e8e96c 100644
--- a/runtime/vm/raw_object.cc
+++ b/runtime/vm/raw_object.cc
@@ -759,6 +759,15 @@ intptr_t RawBigint::VisitBigintPointers(RawBigint* raw_obj,
 }
 
 
+intptr_t RawPixels::VisitPixelsPointers(RawPixels* raw_obj,
+                                        ObjectPointerVisitor* visitor) {
+  // Make sure that we got here with the tagged pointer as this.
+  ASSERT(raw_obj->IsHeapObject());
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return Pixels::InstanceSize();
+}
+
+
 intptr_t RawDouble::VisitDoublePointers(RawDouble* raw_obj,
                                         ObjectPointerVisitor* visitor) {
   // Make sure that we got here with the tagged pointer as this.