Ensure that outgoing constant arguments will be materialized.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index edd92bbaf56d7c49a322af6d263ebc57f23178df..b5fd87d00891a362ff3ecd5d7e219d2c6ef127fe 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -42,6 +42,8 @@ class Value : public Computation {
  public:
   Value() { }
 
+  virtual bool IsConstant() const { return false; }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(Value);
 };
@@ -139,6 +141,8 @@ class ConstantValue: public Value {
  public:
   explicit ConstantValue(const Instance& instance) : instance_(instance) { }
 
+  virtual bool IsConstant() const { return true; }
+
   virtual void Print() const;
 
  private: