Dart_SetReturnValue now accepts and propagates error handles.

runtime/vm/dart_api_impl.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 #ifndef VM_DART_API_IMPL_H_
 #define VM_DART_API_IMPL_H_
 
 #include "vm/allocation.h"
 #include "vm/native_arguments.h"
 #include "vm/object.h"
@@ skipping 144 matching lines @@
     // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.
     RawObject* raw = *(reinterpret_cast<RawObject**>(handle));
     return !raw->IsHeapObject();
   }
 
   // Returns true if the handle holds a Dart Instance.
   static bool IsInstance(Dart_Handle handle) {
     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.
     uword value = *(reinterpret_cast<uword*>(handle));
     return Smi::ValueFromRaw(value);
   }
 
   static intptr_t ClassId(Dart_Handle handle) {
     // TODO(turnidge): Assumes RawObject* is at offset zero.  Fix.
     RawObject* raw = *(reinterpret_cast<RawObject**>(handle));
@@ skipping 116 matching lines @@
     return Api::NewError("%s: Cannot load after Dart_Precompile",              \
                          CURRENT_FUNC);                                        \
   }                                                                            \
 
 #define ASSERT_CALLBACK_STATE(thread)                                         \
   ASSERT(thread->no_callback_scope_depth() == 0)
 
 }  // namespace dart.
 
 #endif  // VM_DART_API_IMPL_H_