[vm, gc] Require a safepoint for heap iteration.

runtime/vm/symbols.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/symbols.h"
 
 #include "vm/handles.h"
 #include "vm/hash_table.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
@@ skipping 351 matching lines @@
       if (obj->IsCanonical() && obj->IsStringInstance()) {
         symbols_->Add(&String::ZoneHandle(zone_, String::RawCast(obj)));
       }
     }
 
    private:
     GrowableArray<String*>* symbols_;
     Zone* zone_;
   };
 
-  SymbolCollector visitor(Thread::Current(), &symbols);
-  isolate->heap()->IterateObjects(&visitor);
+  {
+    HeapIterationScope iteration;
+    SymbolCollector visitor(Thread::Current(), &symbols);
+    iteration.IterateObjects(&visitor);
+  }
 
   // 3. Build a new table from the surviving symbols.
   Array& array = Array::Handle(
       zone, HashTables::New<SymbolTable>(symbols.length() * 4 / 3, Heap::kOld));
   SymbolTable table(zone, array.raw());
   for (intptr_t i = 0; i < symbols.length(); i++) {
     String& symbol = *symbols[i];
     ASSERT(symbol.IsString());
     ASSERT(symbol.IsCanonical());
     bool present = table.Insert(symbol);
@@ skipping 309 matching lines @@
 RawObject* Symbols::GetPredefinedSymbol(intptr_t object_id) {
   ASSERT(IsPredefinedSymbolId(object_id));
   intptr_t i = (object_id - kMaxPredefinedObjectIds);
   if ((i > kIllegal) && (i < Symbols::kMaxPredefinedId)) {
     return symbol_handles_[i]->raw();
   }
   return Object::null();
 }
 
 }  // namespace dart