Fix build breakage.

dart/tests/compiler/dart2js_extra/super_constructor1_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.
 
 String message;
 
 class A {
   int x;
   A(i) : x = i {
-    message += 'A($i)';
+    message = '${message}A($i)';
   }
 }
 
 class B extends A {
   int y;
   B(i) : y = i++, super(i + 5) {
-    message += 'B($i)';
+    message = '${message}B($i)';
   }
 }
 
 class C extends B {
   var z;
   C(i) : super(i * 3), z = i {
-    message += 'C($i)';
+    message = '${message}C($i)';
   }
 }
 
 main() {
   message = '';
   var c = new C(7);
   Expect.equals(27, c.x);
   Expect.equals(21, c.y);
   Expect.equals(7, c.z);
   Expect.equals('A(27)B(22)C(7)', message);
 }