Double the default heap size on 64-bit platforms to account for all pointers taking up twice the am…

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 6dd44192735d1676382984b55633bc05276f161a..2502674d6360f9360668f154bf2e8c8a9b03c95d 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -55,7 +55,8 @@ class Heap {
   };
 
   // Default allocation sizes in MB for the old gen and code heaps.
-  static const intptr_t kHeapSizeInMB = 512;
+  static const intptr_t kHeapSizeInMWords = 128;
+  static const intptr_t kHeapSizeInMB = kHeapSizeInMWords * kWordSize;
   static const intptr_t kCodeHeapSizeInMB = 18;
 
   ~Heap();