Fix a pretty blatant GC bug (was the cause of all the weird crashes on windows).

vm/object.cc

Index: vm/object.cc
===================================================================
--- vm/object.cc	(revision 6711)
+++ vm/object.cc	(working copy)
@@ -8601,9 +8601,10 @@
 
 void GrowableObjectArray::Grow(intptr_t new_capacity, Heap::Space space) const {
   ASSERT(new_capacity > Capacity());
-  Array& contents = Array::Handle(data());
-  StorePointer(&(raw_ptr()->data_),
-               Array::Grow(contents, new_capacity, space));
+  const Array& contents = Array::Handle(data());
+  const Array& new_contents =
+      Array::Handle(Array::Grow(contents, new_capacity, space));
+  StorePointer(&(raw_ptr()->data_), new_contents.raw());
 }