Implement ?? as compile-time constant in VM.

runtime/vm/parser.cc

 // 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.
 
 #include "vm/parser.h"
 #include "vm/flags.h"
 
 #ifndef DART_PRECOMPILED_RUNTIME
 
 #include "lib/invocation_mirror.h"
@@ skipping 10405 matching lines @@
           BinaryOpNode* old = lhs->AsBinaryOpNode();
           BinaryOpWithMask32Node* binop = new(Z) BinaryOpWithMask32Node(
               old->token_pos(), old->kind(), old->left(), old->right(), val);
           return binop;
         }
       }
     }
   }
   if (binary_op == Token::kIFNULL) {
     // Handle a ?? b.
+    if ((lhs->EvalConstExpr() != NULL) && (rhs->EvalConstExpr() != NULL)) {
+      Instance& expr_value = Instance::ZoneHandle(Z);
+      if (!GetCachedConstant(op_pos, &expr_value)) {
+        expr_value = EvaluateConstExpr(lhs->token_pos(), lhs).raw();
+        if (expr_value.IsNull()) {
+          expr_value = EvaluateConstExpr(rhs->token_pos(), rhs).raw();
+        }
+        CacheConstantValue(op_pos, expr_value);
+      }
+      return new(Z) LiteralNode(op_pos, expr_value);
+    }
+
     LetNode* result = new(Z) LetNode(op_pos);
     LocalVariable* left_temp = result->AddInitializer(lhs);
+    left_temp->set_is_final();
     const intptr_t no_pos = Token::kNoSourcePos;
     LiteralNode* null_operand =
         new(Z) LiteralNode(no_pos, Object::null_instance());
     LoadLocalNode* load_left_temp = new(Z) LoadLocalNode(no_pos, left_temp);
     ComparisonNode* null_compare =
         new(Z) ComparisonNode(no_pos,
                               Token::kNE_STRICT,
                               load_left_temp,
                               null_operand);
     result->AddNode(new(Z) ConditionalExprNode(op_pos,
@@ skipping 3863 matching lines @@
     const ArgumentListNode& function_args,
     const LocalVariable* temp_for_last_arg,
     bool is_super_invocation) {
   UNREACHABLE();
   return NULL;
 }
 
 }  // namespace dart
 
 #endif  // DART_PRECOMPILED_RUNTIME