Implement ?? as compile-time constant in VM.

runtime/vm/ast.h

Index: runtime/vm/ast.h
diff --git a/runtime/vm/ast.h b/runtime/vm/ast.h
index d1e62c9326508ef56679e930195fa0ae2f86dd6f..494d76876adec994bc17e4979625632a6cda15ea 100644
--- a/runtime/vm/ast.h
+++ b/runtime/vm/ast.h
@@ -328,6 +328,9 @@ class LetNode : public AstNode {
   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_; }