Make ?? a compile-time constant operator.

runtime/vm/ast.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/ast.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
 #include "vm/object_store.h"
 #include "vm/resolver.h"
@@ skipping 294 matching lines @@
     case Token::kSUB:
     case Token::kMUL:
     case Token::kDIV:
     case Token::kMOD:
     case Token::kTRUNCDIV:
     case Token::kBIT_OR:
     case Token::kBIT_XOR:
     case Token::kBIT_AND:
     case Token::kSHL:
     case Token::kSHR:
+    case Token::kIFNULL:
       return this->left()->IsPotentiallyConst() &&
           this->right()->IsPotentiallyConst();
     default:
       UNREACHABLE();
       return false;
   }
 }
 
 
 const Instance* BinaryOpNode::EvalConstExpr() const {
   const Instance* left_val = this->left()->EvalConstExpr();
   if (left_val == NULL) {
     return NULL;
   }
-  if (!left_val->IsNumber() && !left_val->IsBool() && !left_val->IsString()) {
+  if (!left_val->IsNumber() && !left_val->IsBool() && !left_val->IsString() &&
+      kind_ != Token::kIFNULL) {
     return NULL;
   }
   const Instance* right_val = this->right()->EvalConstExpr();
   if (right_val == NULL) {
     return NULL;
   }
   switch (kind_) {
     case Token::kADD:
       if (left_val->IsString()) {
         return right_val->IsString() ? left_val : NULL;
@@ skipping 24 matching lines @@
           right_val->IsInteger()) {
         return right_val;
       }
       return NULL;
     case Token::kOR:
     case Token::kAND:
       if (left_val->IsBool() && right_val->IsBool()) {
         return left_val;
       }
       return NULL;
+    case Token::kIFNULL:
+      if (left_val->IsNull()) return right_val;
+      return left_val;
     default:
       UNREACHABLE();
       return NULL;
   }
   return NULL;
 }
 
 
 AstNode* UnaryOpNode::UnaryOpOrLiteral(intptr_t token_pos,
                                        Token::Kind kind,
@@ skipping 379 matching lines @@
   if (result.IsError() || result.IsNull()) {
     // TODO(turnidge): We could get better error messages by returning
     // the Error object directly to the parser.  This will involve
     // replumbing all of the EvalConstExpr methods.
     return NULL;
   }
   return &Instance::ZoneHandle(Instance::Cast(result).raw());
 }
 
 }  // namespace dart