Rename String.concat to operator+.

tests/language/constructor7_test.dart

 // Copyright (c) 2011, 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 constructors and initializers.
 
 // Expect the initializer expressions E(i) to be evaluated
 // in the order 1, 2, 3, ...
 // This test has no inheritance but many fields to flush out issues with
 // ordering of fields.
 
 String trace = "";
 
 int E(int i) {
-  trace = trace.concat("$i-");
+  trace += "$i-";
   return i;
 }
 
 class A {
   var j;      // Names are in reverse order to detect sorting by name...
   var i = 0;  // Initialized odd/even to detect these inits affecting order.
   var h;
   var g = 0;
   var f;
   var e = 0;
@@ skipping 28 matching lines @@
     Expect.equals(8, h);
     Expect.equals(9, i);
     Expect.equals(10, j);
   }
 }
 
 main() {
   var x = new A();
   Expect.equals('1-2-3-4-5-6-7-8-9-10-', trace);
 }