Fix analyzer's treatment of mixin constructors.

tests/language/mixin_super_constructor_positionals_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 Base {
   int i, j;
-  Base.ctor(int this.i, [int this.j = 10]);
+  Base.ctor(int this.i,
+            [                  /// 01: compile-time error
+            int this.j
+            ]                  /// 01: continued
+           );
 }
 
 abstract class M {
   get i;
   get j;
   int k = 42;
   foo() => i + j;
 }
 
 class C extends Base with M {
   int l = 131;
   C.foo() : super.ctor(1, 13);
-  C.bar() : super.ctor(1);
+  C.bar() : super.ctor(1);     /// 01: continued
 }
 
 main() {
   C c1 = new C.foo();
   Expect.equals(1, c1.i);
   Expect.equals(13, c1.j);
   Expect.equals(14, c1.foo());
   Expect.equals(42, c1.k);
   Expect.equals(131, c1.l);
-  C c2 = new C.bar();
-  Expect.equals(1, c2.i);
-  Expect.equals(10, c2.j);
-  Expect.equals(11, c2.foo());
-  Expect.equals(42, c2.k);
-  Expect.equals(131, c2.l);
+  C c2 = new C.bar();          /// 01: continued
 }