Fix analyzer's treatment of mixin constructors.

tests/language/mixin_forwarding_constructor2_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:expect/expect.dart";
 
 abstract class Mixin1 {
   var mixin1Field = 1;
 }
 
 abstract class Mixin2 {
   var mixin2Field = 2;
 }
 
 class A {
   var superField;
-  A([this.superField = 3]);
+  A() : superField = 3;
 }
 
 class B extends A with Mixin1, Mixin2 {
   var field = 4;
 }
 
 main() {
   var b = new B();
   Expect.equals(1, b.mixin1Field);
   Expect.equals(2, b.mixin2Field);
   Expect.equals(3, b.superField);
   Expect.equals(4, b.field);
 }