Fix for issue 29350 - VM Crashes when covariant is used incorrectly.

tests/standalone/regress_29350_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.
+ */
+
+// 'covariant' when used incorrectly should report an error and not crash
+// the VM.
+
+typedef bool Test(covariant num arg); //# none: compile-time error
+
+void main() {
+  Test t = (int value) => false;
+}