Rename String.concat to operator+.

tests/language/constructor3_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, ...
 // Each expression must be evaluated exactly once.
 
 String trace = "";
 
 int E(int i) {
-  trace = trace.concat("$i-");
+  trace += "$i-";
   return i;
 }
 
 class A {
   var a1;
   A(x, y) : a1 = E(3) {
     Expect.equals(1, x);
     Expect.equals(2, y);
     E(5);
   }
 }
 
 class B extends A {
   var b1;
   B(x) : super(E(1), E(2)), b1 = E(4) {
     // Implicit super call to A's body happens here.
     Expect.equals(4, b1);
     E(6);
   }
 }
 
 main() {
   var b = new B(0);
   Expect.equals("1-2-3-4-5-6-", trace);
 }