Cleanup handling of NullType in type propagation.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 10752)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -296,6 +296,11 @@
  public:
   Value() { }
 
+  // Returns true if the value is constant null.
+  bool IsConstantNull() const;
+
+  // Reminder: The type of the constant null is the bottom type, which is more
+  // specific than any type.
   bool CompileTypeIsMoreSpecificThan(const AbstractType& dst_type) const;
 
   virtual void RemoveFromUseList() = 0;
@@ -2880,9 +2885,13 @@
 class FlowGraphVisitor : public ValueObject {
  public:
   explicit FlowGraphVisitor(const GrowableArray<BlockEntryInstr*>& block_order)
-      : block_order_(block_order) { }
+      : block_order_(block_order), current_iterator_(NULL) { }
   virtual ~FlowGraphVisitor() { }
 
+  ForwardInstructionIterator* current_iterator() const {
+    return current_iterator_;
+  }
+
   // Visit each block in the block order, and for each block its
   // instructions in order from the block entry to exit.
   virtual void VisitBlocks();
@@ -2903,6 +2912,7 @@
 
  protected:
   const GrowableArray<BlockEntryInstr*>& block_order_;
+  ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);