Heap::AllocateExternal may trigger GC, so fix String::MakeExternal.

runtime/vm/dart_api_state.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_DART_API_STATE_H_
 #define VM_DART_API_STATE_H_
 
 #include "include/dart_api.h"
 
 #include "platform/thread.h"
@@ skipping 951 matching lines @@
     intptr_t external_size) {
   ApiState* state = isolate->api_state();
   ASSERT(state != NULL);
   FinalizablePersistentHandle* ref = is_prologue ?
       state->prologue_weak_persistent_handles().AllocateHandle() :
       state->weak_persistent_handles().AllocateHandle();
   ref->SetPrologueWeakPersistent(is_prologue);
   ref->set_raw(object);
   ref->set_peer(peer);
   ref->set_callback(callback);
+  // This may trigger GC, so it must be called last.
   ref->SetExternalSize(external_size, isolate);
   return ref;
 }
 
 }  // namespace dart
 
 #endif  // VM_DART_API_STATE_H_