Fix analyzer's treatment of mixin constructors.

tests/language/mixin_forwarding_constructor4_test.dart

+// Copyright (c) 2014, 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.
+
+// Verify that when no constructors are forwarded from the base class
+// through a mixin, it is always an error; the mixin does not acquire an
+// implicit default constructor.
+
+abstract class Mixin {}
+
+class Base {
+  Base(
+      {x}        /// 01: compile-time error
+      {x}        /// 02: compile-time error
+      {x}        /// 03: compile-time error
+    );
+}
+
+class C extends Base with Mixin {
+  C();           /// 02: continued
+  C() : super(); /// 03: continued
+}
+
+main() {
+  new C();
+}