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

runtime/vm/object_test.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 "platform/globals.h"
 
 #include "vm/assembler.h"
 #include "vm/class_finalizer.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_api_impl.h"
@@ skipping 4049 matching lines @@
   }
 
  private:
   GrowableArray<Object*>* objects_;
 };
 
 ISOLATE_UNIT_TEST_CASE(PrintJSON) {
   Heap* heap = Isolate::Current()->heap();
   heap->CollectAllGarbage();
   GrowableArray<Object*> objects;
-  ObjectAccumulator acc(&objects);
-  heap->IterateObjects(&acc);
+  {
+    HeapIterationScope iteration(Thread::Current());
+    ObjectAccumulator acc(&objects);
+    iteration.IterateObjects(&acc);
+  }
   for (intptr_t i = 0; i < objects.length(); ++i) {
     JSONStream js;
     objects[i]->PrintJSON(&js, false);
     EXPECT_SUBSTRING("\"type\":", js.ToCString());
   }
 }
 
 ISOLATE_UNIT_TEST_CASE(PrintJSONPrimitives) {
   char buffer[1024];
   Isolate* isolate = Isolate::Current();
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   // utf32->utf16 conversion.
   int32_t char_codes[] = {0,      0x0a,   0x0d,   0x7f,   0xff,
                           0xffff, 0xd800, 0xdc00, 0xdbff, 0xdfff};
 
   const String& str =
       String::Handle(String::FromUTF32(char_codes, ARRAY_SIZE(char_codes)));
   EXPECT(str.Equals(char_codes, ARRAY_SIZE(char_codes)));
 }
 
 }  // namespace dart