- Fix use of String operator + in language and corelib tests.

tests/language/src/Constructor2Test.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 += "$i-";
+  trace = trace.concat("$i-");
   return i;
 }
 
 class A {
   var a1;
   A(x, y) : a1 = E(4) {
     Expect.equals(2, x);
     Expect.equals(3, y);
     Expect.equals(4, a1);
     E(6);
   }
 }
 
 class B extends A {
   var b1, b2;
 
   B(x) : b1 = E(1), super(E(2), E(3)), b2 = E(5) {
     Expect.equals(1, b1);
     Expect.equals(5, b2);
     E(7);
   }
 }
 
 main() {
   var b = new B(0);
   Expect.equals("1-2-3-4-5-6-7-", trace);
 }