Implement type variables in mixin applications.

pkg/front_end/test/fasta/mixin.dart

 // Copyright (c) 2016, 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 B extends Object with M1, M2 {
+  B(value);
+}
+
 abstract class M1 {
   m() => print("M1");
 }
 
 abstract class M2 {
   m() => print("M2");
 }
 
 class C extends Object with M1, M2 {
+  C(value);
+}
+
+abstract class G1<T> {
+  m() => print(T);
+}
+
+class D<S> extends Object with G1<S> {
 }
 
 main() {
-  new C().m();
+  new B(null).m();
+  new C(null).m();
+  new D().m();
+  new D<int>().m();
+  new D<List<int>>().m();
 }