Fix crash in inference

tests/language/regress_30121_test.dart

Index: tests/language/regress_30121_test.dart
diff --git a/tests/language/regress_30121_test.dart b/tests/language/regress_30121_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..99451f5eb0f629ac783666fe8ef50e51150fd219
--- /dev/null
+++ b/tests/language/regress_30121_test.dart
@@ -0,0 +1,18 @@
+// 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();
+}