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

tests/language/const_constructor_mixin_test.dart

Index: tests/language/const_constructor_mixin_test.dart
===================================================================
--- tests/language/const_constructor_mixin_test.dart	(revision 0)
+++ tests/language/const_constructor_mixin_test.dart	(revision 0)
@@ -0,0 +1,18 @@
+// 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.
+
+class Mixin {
+}
+
+class A {
+  const A(foo);
+}
+
+class B extends A with Mixin {
+  const B(foo) : super(foo);
+}
+
+main() {
+  var a = const B(42);
+}