Rename String.concat to operator+.

tests/language/super_field_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 for testing super field access.
 
 
 class A {
   A() {
     city = "Bern";
   }
   String greeting() {
     return "Gruezi";
   }
   String city;
 }
 
 
 class B extends A {
   B() : super() {}
   String greeting() {
-    return "Hola ".concat(super.greeting());
+    return "Hola " + super.greeting();
   }
 }
 
 
 class C extends B {
   C() : super() {}
   String greeting() {
-    return "Servus ".concat(super.greeting());
+    return "Servus " + super.greeting();
   }
   String get city {
-    return "Basel ".concat(super.city);
+    return "Basel " + super.city;
   }
 }
 
 
 class SuperFieldTest {
   static testMain() {
     A a = new A();
     B b = new B();
     C c = new C();
     Expect.equals("Gruezi", a.greeting());
     Expect.equals("Hola Gruezi", b.greeting());
     Expect.equals("Servus Hola Gruezi", c.greeting());
 
     Expect.equals("Bern", a.city);
     Expect.equals("Bern", b.city);
     Expect.equals("Basel Bern", c.city);
     c.city = "Zurich";
     Expect.equals("Basel Zurich", c.city);
   }
 }
 
 main() {
   SuperFieldTest.testMain();
 }