Fix crash in inference

tests/language/regress_30121_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.
+
+class Mock {
+  noSuchMethod(i) {}
+}
+
+class Foo {
+  int call() => 1;
+}
+
+class MockFoo extends Mock implements Foo {}
+
+main() {
+  var foo = new MockFoo();
+  foo();
+}