Use the return value of vm native methods to set the return value,

vm/native_arguments.h

 // Copyright (c) 2012, 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_NATIVE_ARGUMENTS_H_
 #define VM_NATIVE_ARGUMENTS_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/stub_code.h"
 
 namespace dart {
 
 // Forward declarations.
+class BootstrapNatives;
 class Isolate;
 class Object;
 class RawObject;
 
 
 #if defined(TESTING) || defined(DEBUG)
 
 #if defined(TARGET_OS_WINDOWS)
 // The compiler may dynamically align the stack on Windows, so do not check.
 #define CHECK_STACK_ALIGNMENT { }
@@ skipping 44 matching lines @@
   static intptr_t isolate_offset() {
     return OFFSET_OF(NativeArguments, isolate_);
   }
   static intptr_t argc_offset() { return OFFSET_OF(NativeArguments, argc_); }
   static intptr_t argv_offset() { return OFFSET_OF(NativeArguments, argv_); }
   static intptr_t retval_offset() {
     return OFFSET_OF(NativeArguments, retval_);
   }
 
  private:
+  friend class BootstrapNatives;
+
+  // Since this function is passed a RawObject directly, we need to be
+  // exceedingly careful when we use it.  If there are any other side
+  // effects in the statement that may cause GC, it could lead to
+  // bugs.
+  void SetReturnUnsafe(RawObject* value) const;
+
   Isolate* isolate_;  // Current isolate pointer.
   int argc_;  // Number of arguments passed to the runtime call.
   RawObject*(*argv_)[];  // Pointer to an array of arguments to runtime call.
   RawObject** retval_;  // Pointer to the return value area.
 };
 
 }  // namespace dart
 
 #endif  // VM_NATIVE_ARGUMENTS_H_