Implement const expressions for local variables

runtime/vm/scopes.h

Index: runtime/vm/scopes.h
===================================================================
--- runtime/vm/scopes.h	(revision 14713)
+++ runtime/vm/scopes.h	(working copy)
@@ -28,6 +28,7 @@
       name_(name),
       owner_(NULL),
       type_(type),
+      const_value_(NULL),
       is_final_(false),
       is_captured_(false),
       is_invisible_(false),
@@ -71,6 +72,20 @@
     is_invisible_ = value;
   }
 
+  bool IsConst() const {
+    return const_value_ != NULL;
+  }
+
+  void SetConstValue(const Instance& value) {
+    ASSERT(value.IsZoneHandle());
+    const_value_ = &value;
+  }
+
+  const Instance* ConstValue() const {
+    ASSERT(IsConst());
+    return const_value_;
+  }
+
   bool Equals(const LocalVariable& other) const;
 
   // Map the frame index to a bit-vector index.  Assumes the variable is
@@ -88,6 +103,8 @@
 
   const AbstractType& type_;  // Declaration type of local variable.
 
+  const Instance* const_value_;   // NULL or compile-time const value.
+
   bool is_final_;  // If true, this variable is readonly.
   bool is_captured_;  // If true, this variable lives in the context, otherwise
                       // in the stack frame.