Cleanup handling of NullType in type propagation.

runtime/vm/intermediate_language.h

 // 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.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 277 matching lines @@
 
  protected:
   EmbeddedArray<Value*, N> inputs_;
 };
 
 
 class Value : public TemplateComputation<0> {
  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;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Value);
 };
 
 
 // Functions defined in all concrete computation classes.
@@ skipping 2567 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(Environment);
 };
 
 
 // Visitor base class to visit each instruction and computation in a flow
 // graph as defined by a reversed list of basic blocks.
 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();
 
   // Visit functions for instruction and computation classes, with empty
   // default implementations.
 #define DECLARE_VISIT_COMPUTATION(ShortName, ClassName)                        \
   virtual void Visit##ShortName(ClassName* comp, BindInstr* instr) { }
 
 #define DECLARE_VISIT_INSTRUCTION(ShortName)                                   \
   virtual void Visit##ShortName(ShortName##Instr* instr) { }
 
   FOR_EACH_COMPUTATION(DECLARE_VISIT_COMPUTATION)
   FOR_EACH_INSTRUCTION(DECLARE_VISIT_INSTRUCTION)
 
 #undef DECLARE_VISIT_COMPUTATION
 #undef DECLARE_VISIT_INSTRUCTION
 
  protected:
   const GrowableArray<BlockEntryInstr*>& block_order_;
+  ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_