Make ?? a compile-time constant operator.

runtime/vm/ast.cc

Index: runtime/vm/ast.cc
diff --git a/runtime/vm/ast.cc b/runtime/vm/ast.cc
index 48bd0534e0b425094f3e700caaeeb38ae81486c7..6ce0703e7141d3324c69b5b5dd24acc24e350578 100644
--- a/runtime/vm/ast.cc
+++ b/runtime/vm/ast.cc
@@ -312,6 +312,7 @@ bool BinaryOpNode::IsPotentiallyConst() const {
     case Token::kBIT_AND:
     case Token::kSHL:
     case Token::kSHR:
+    case Token::kIFNULL:
       return this->left()->IsPotentiallyConst() &&
           this->right()->IsPotentiallyConst();
     default:
@@ -326,7 +327,8 @@ const Instance* BinaryOpNode::EvalConstExpr() const {
   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();
@@ -371,6 +373,9 @@ const Instance* BinaryOpNode::EvalConstExpr() const {
         return left_val;
       }
       return NULL;
+    case Token::kIFNULL:
+      if (left_val->IsNull()) return right_val;
+      return left_val;
     default:
       UNREACHABLE();
       return NULL;