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

tests/language/mixin_with_two_implicit_constructors_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";
+
+class A {
+  var field;
+  A.bar() : field = 1;
+  A() : field = 2;
+}
+
+class Mixin {
+}
+
+class B extends A with Mixin {
+}
+
+main() {
+  Expect.equals(2, new B().field);
+  Expect.throws(() => new B.bar(), (e) => e is NoSuchMethodError);
+}