Rename String.concat to operator+.

tests/language/constructor_named_arguments_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.
 // Dart test program for default constructors.
 
 String message;
 
 foo() {
-  message = message.concat('foo');
+  message += 'foo';
   return 1;
 }
 
 bar() {
-  message = message.concat('bar');
+  message += 'bar';
   return 2;
 }
 
 class X {
   var i;
   var j;
   X({a: 'defa', b: 'defb'}) : this.i = a, this.j = b;
   X.foo() : this(b: 1, a: 2);
   X.bar() : this(
                      1,  /// 01: runtime error
@@ skipping 18 matching lines @@
   test(new X.hest(), 'defa', 'defb');
 
   message = '';
   new X.fisk();
   Expect.equals('barfoo', message);
 
   message = '';
   new X.naebdyr();
   Expect.equals('foobar', message);
 }