Avoid crash for type variables in mixed in fields.

tests/language/closure_in_field_test.dart

+// Copyright (c) 2017, 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 handling of type variable in field initializers.
+
+import 'package:expect/expect.dart';
+
+class Mixin<S> {
+  var field = (S s) => null;
+}
+
+class Class<T> extends Object with Mixin<T> {}
+
+void main() {
+  Expect.isTrue(test(new Mixin<int>()));
+  Expect.isFalse(test(new Mixin<String>())); //# 01: ok
+  Expect.isTrue(test(new Class<int>()));
+  Expect.isFalse(test(new Class<String>())); //# 02: ok
+}
+
+test(o) => o.field is dynamic Function(int);