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

runtime/vm/heap.cc

 // 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.
 
 #include "vm/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 74 matching lines @@
     if (addr == 0) {
       OS::PrintErr("Exhausted heap space, trying to allocate %" Pd " bytes.\n",
                    size);
       return 0;
     }
   }
   return addr;
 }
 
 void Heap::AllocateExternal(intptr_t size, Space space) {
+  ASSERT(isolate()->no_gc_scope_depth() == 0);
   if (space == kNew) {
     new_space_->AllocateExternal(size);
     if (new_space_->ExternalInWords() > (FLAG_new_gen_ext_limit * MBInWords)) {
       // Attempt to free some external allocation by a scavenge. (If the total
       // remains above the limit, next external alloc will trigger another.)
       CollectGarbage(kNew);
     }
   } else {
     ASSERT(space == kOld);
     old_space_->AllocateExternal(size);
@@ skipping 473 matching lines @@
     heap->DisableGrowthControl();
 }
 
 
 NoHeapGrowthControlScope::~NoHeapGrowthControlScope() {
     Heap* heap = reinterpret_cast<Isolate*>(isolate())->heap();
     heap->SetGrowthControlState(current_growth_controller_state_);
 }
 
 }  // namespace dart