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

tests/language/inference_mixin_field_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 Mixin {
+  var field;
+  createIt() {
+    if (field == null) field = 42;
+  }
+}
+
+class A {
+  A(foo);
+}
+
+class B extends A with Mixin {
+  // Because [super] references a synthesized constructor, dart2js
+  // used to not see the null assignment to it.
+  B(foo) : super(foo);
+}
+
+main() {
+  var a = new B(42);
+  a.createIt();
+  Expect.equals(42, a.field);
+}