Rename String.concat to operator+.

tests/compiler/dart2js_extra/to_string_test.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 A extends Object {
 }
 
 class Concater {
   final x;
   final y;
   Concater(x, y) : this.x = x, this.y = y;
-  add() => x.concat(y.toString());
+  add() => x + y.toString();
 }
 
 test(expected, x) {
   Expect.equals(expected, x.toString());
   Expect.equals(expected, new Concater("", x).add());
 }
 
 main() {
   test("Instance of 'Object'", new Object());
   test("Instance of 'A'", new A());
   test("[]", []);
   test("1", 1);
 }