Dart_SetReturnValue now accepts and propagates error handles.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
diff --git a/runtime/vm/dart_api_impl.h b/runtime/vm/dart_api_impl.h
index ee36410d6512685881ef23da9a470753b8d1cdac..9820417debabd26a98a6591b0aab8450d486dd3d 100644
--- a/runtime/vm/dart_api_impl.h
+++ b/runtime/vm/dart_api_impl.h
@@ -162,6 +162,11 @@ class Api : AllStatic {
     return (ClassId(handle) >= kInstanceCid);
   }
 
+  // Returns true if the handle holds an Error.
+  static bool IsError(Dart_Handle handle) {
+    return RawObject::IsErrorClassId(ClassId(handle));
+  }
+
   // Returns the value of a Smi.
   static intptr_t SmiValue(Dart_Handle handle) {
     // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.