Add Heap::isolate_ to simplify code.

runtime/vm/heap.h

Index: runtime/vm/heap.h
===================================================================
--- runtime/vm/heap.h	(revision 35410)
+++ runtime/vm/heap.h	(working copy)
@@ -117,18 +117,18 @@
   bool StubCodeContains(uword addr) const;
 
   // Visit all pointers.
-  void IteratePointers(ObjectPointerVisitor* visitor);
+  void IteratePointers(ObjectPointerVisitor* visitor) const;
 
   // Visit all pointers in the space.
-  void IterateNewPointers(ObjectPointerVisitor* visitor);
-  void IterateOldPointers(ObjectPointerVisitor* visitor);
+  void IterateNewPointers(ObjectPointerVisitor* visitor) const;
+  void IterateOldPointers(ObjectPointerVisitor* visitor) const;
 
   // Visit all objects.
-  void IterateObjects(ObjectVisitor* visitor);
+  void IterateObjects(ObjectVisitor* visitor) const;
 
   // Visit all object in the space.
-  void IterateNewObjects(ObjectVisitor* visitor);
-  void IterateOldObjects(ObjectVisitor* visitor);
+  void IterateNewObjects(ObjectVisitor* visitor) const;
+  void IterateOldObjects(ObjectVisitor* visitor) const;
 
   // Find an object by visiting all pointers in the specified heap space,
   // the 'visitor' is used to determine if an object is found or not.
@@ -249,6 +249,8 @@
     return old_space_->PrintHeapMapToJSONStream(isolate, stream);
   }
 
+  Isolate* isolate() const { return isolate_; }
+
  private:
   class GCStats : public ValueObject {
    public:
@@ -282,7 +284,9 @@
 
   static const intptr_t kNewAllocatableSize = 256 * KB;
 
-  Heap(intptr_t max_new_gen_words, intptr_t max_old_gen_words);
+  Heap(Isolate* isolate,
+       intptr_t max_new_gen_words,
+       intptr_t max_old_gen_words);
 
   uword AllocateNew(intptr_t size);
   uword AllocateOld(intptr_t size, HeapPage::PageType type);
@@ -298,6 +302,8 @@
   // of this heap.
   void GetMergedAddressRange(uword* start, uword* end) const;
 
+  Isolate* isolate_;
+
   // The different spaces used for allocation.
   Scavenger* new_space_;
   PageSpace* old_space_;