Optimize identical-comparisons based on propagated type information.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 28802)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -366,7 +366,7 @@
 
 
 // NotNullConstrainedCompileType represents not-null constraint applied to
-// the source compile type. Result is non-nullable version of the incomming
+// the source compile type. Result is non-nullable version of the incoming
 // compile type. It is used to represent compile type propagated downwards
 // from strict comparison with the null constant.
 class NotNullConstrainedCompileType : public ConstrainedCompileType {
@@ -506,6 +506,8 @@
   bool Equals(Value* other) const;
 
  private:
+  friend class FlowGraphPrinter;
+
   Definition* definition_;
   Value* previous_use_;
   Value* next_use_;
@@ -1752,6 +1754,7 @@
 
  protected:
   friend class RangeAnalysis;
+  friend class Value;
 
   Range* range_;
   CompileType* type_;