Support general handling of type variables by substituting types into the current context.

tests/language/generic_field_mixin2_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.
+
+// Test that generic types in mixins are handled.
+
+import 'package:expect/expect.dart';
+
+class M<T> {
+  var field = T;
+}
+class A<U> {}
+class C1<V> = Object with M<V>;
+class C2 = Object with M<int>;
+class C3 = Object with M<String>;
+
+main() {
+  Expect.equals(int, new C1<int>().field);
+  Expect.equals(String, new C1<String>().field);
+
+  Expect.equals(int, new C2().field);
+
+  Expect.equals(String, new C3().field);
+}