Compute correct constructor type for synthesized mixin constructors.

tests/language/typevariable_substitution2_test.dart

Index: tests/language/typevariable_substitution2_test.dart
diff --git a/tests/language/typevariable_substitution2_test.dart b/tests/language/typevariable_substitution2_test.dart
new file mode 100644
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+++ b/tests/language/typevariable_substitution2_test.dart
@@ -0,0 +1,26 @@
+// 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.
+
+// Test that mixins don't interfere with type variable substitution.
+
+import 'checked_mode_helper.dart';
+
+class B<T> {
+  B(T x);
+}
+
+class M {}
+
+class A<T> extends B<T> with M {
+  A(T x) : super(x);  // This line must be warning free.
+}
+
+class C<T> = B<T> with M;
+
+
+main() {
+  new A(null);
+  new C<String>(''); /// 01: ok
+  checkDynamicTypeError(() => new C<String>(0)); /// 02: static type warning
+}