Support closures inside initializers.

tests/language/constructor6_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 {
   // f closes-over arg.  arg needs to be preserved while b2 is initialized.
   A(arg)
       : a = E(arg += 1)
       , f = (() => E(arg += 10)) {
     // b2 should be initialized between the above initializers and the following
     // statements.
@@ skipping 18 matching lines @@
 }
 
 class C extends B {
   C() : super(10, 20);
 }
 
 main() {
   var c = new C();
   Expect.equals("10-1001-20-1011-1111-1121-11-21-1131-", trace);
 }