Remove support for String operator + from dart2js.

dart/tests/language/constructor5_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.
 
 // Super initializer and super constructor body are executed in with the same
 // bindings.
 
 String trace = "";
 
 int E(int i) {
-  trace += "$i-";
+  trace = "$trace$i-";
   return i;
 }
 
 class A {
   A([arg1 = 100, arg2 = 200]) : a1 = E(arg1++), a2 = E(arg2++) {
     // b2 should be initialized between the above initializers and the following
     // statements.
     E(arg1);  // 101
     E(arg2);  // 51
   }
@@ skipping 18 matching lines @@
 
 main() {
   var c = new C();
   Expect.equals(100, c.a1);
   Expect.equals(50, c.a2);
   Expect.equals(10, c.b1);
   Expect.equals(20, c.b2);
 
   Expect.equals("10-100-50-20-101-51-11-21-", trace);
 }