Rewrite how we handle synthesized constructors in the compiler. This was motivated by issue https:/…

tests/language/mixin_forwarding_constructor1_test.dart

Index: tests/language/mixin_forwarding_constructor1_test.dart
===================================================================
--- tests/language/mixin_forwarding_constructor1_test.dart	(revision 0)
+++ tests/language/mixin_forwarding_constructor1_test.dart	(revision 0)
@@ -0,0 +1,31 @@
+// 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(foo) : superField = foo;
+}
+
+class B extends A with Mixin1, Mixin2 {
+  var field = 4;
+  B(unused) : super(3);
+}
+
+main() {
+  var b = new B(null);
+  Expect.equals(1, b.mixin1Field);
+  Expect.equals(2, b.mixin2Field);
+  Expect.equals(3, b.superField);
+  Expect.equals(4, b.field);
+}