Implement ?? as compile-time constant in VM.

runtime/vm/ast.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_AST_H_
 #define VM_AST_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/growable_array.h"
@@ skipping 307 matching lines @@
 };
 
 
 class LetNode : public AstNode {
  public:
   explicit LetNode(intptr_t token_pos);
 
   LocalVariable* TempAt(intptr_t i) const { return vars_[i]; }
   AstNode* InitializerAt(intptr_t i) const { return initializers_[i]; }
 
+  virtual bool IsPotentiallyConst() const;
+  virtual const Instance* EvalConstExpr() const;
+
   LocalVariable* AddInitializer(AstNode* node);
 
   const GrowableArray<AstNode*>& nodes() const { return nodes_; }
 
   void AddNode(AstNode* node) { nodes_.Add(node); }
 
   intptr_t num_temps() const {
     return vars_.length();
   }
 
@@ skipping 1686 matching lines @@
   const intptr_t try_index_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(InlinedFinallyNode);
 };
 
 }  // namespace dart
 
 #undef DECLARE_COMMON_NODE_FUNCTIONS
 
 #endif  // VM_AST_H_