Remove support for String operator + from dart2js.

dart/lib/compiler/implementation/compile_time_constants.dart

 // 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.
 
 class Constant implements Hashable {
   const Constant();
 
   bool isNull() => false;
   bool isBool() => false;
   bool isTrue() => false;
@@ skipping 851 matching lines @@
       if (folded === null) error(send);
       return folded;
     } else if (send.isOperator && !send.isPostfix) {
       assert(send.argumentCount() == 1);
       Constant left = evaluate(send.receiver);
       Constant right = evaluate(send.argumentsNode.nodes.head);
       Operator op = send.selector.asOperator();
       Constant folded = null;
       switch (op.source.stringValue) {
         case "+":
-          if (left.isString() && !right.isString()) {
-            // At the moment only compile-time concatenation of two strings is
-            // allowed.
-            error(send);
-          }
           folded = const AddOperation().fold(left, right);
           break;
         case "-":
           folded = const SubtractOperation().fold(left, right);
           break;
         case "*":
           folded = const MultiplyOperation().fold(left, right);
           break;
         case "/":
           folded = const DivideOperation().fold(left, right);
@@ skipping 281 matching lines @@
       Constant fieldValue = fieldValues[field];
       if (fieldValue === null) {
         // Use the default value.
         fieldValue = compiler.compileVariable(field);
       }
       jsNewArguments.add(fieldValue);
     });
     return jsNewArguments;
   }
 }