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

tests/language/mixin_typedef_constructor_test.dart

Index: tests/language/mixin_typedef_constructor_test.dart
===================================================================
--- tests/language/mixin_typedef_constructor_test.dart	(revision 0)
+++ tests/language/mixin_typedef_constructor_test.dart	(revision 0)
@@ -0,0 +1,22 @@
+// 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";
+
+class A {
+  var field;
+  A(this.field);
+}
+
+class Mixin {
+  var mixinField = 54;
+}
+
+typedef MyClass = A with Mixin;
+
+main() {
+  var a = new MyClass(42);
+  Expect.equals(42, a.field);
+  Expect.equals(54, a.mixinField);
+}