Add tests demonstrating bugs #30547 and #30552.

tests/language_2/multiple_interface_inheritance_test.dart

Index: tests/language_2/multiple_interface_inheritance_test.dart
diff --git a/tests/language_2/multiple_interface_inheritance_test.dart b/tests/language_2/multiple_interface_inheritance_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..137814cfede8aa3f48e2048bda0e6b0abd0225cb
--- /dev/null
+++ b/tests/language_2/multiple_interface_inheritance_test.dart
@@ -0,0 +1,36 @@
+// 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.
+
+abstract class I1 {
+  void f(int i);
+}
+
+abstract class I2 {
+  void f(Object o);
+}
+
+abstract class C implements I1, I2 {}
+
+class D extends C {
+  void f(Object o) {}
+}
+
+abstract class E implements I2, I1 {}
+
+class F extends E {
+  void f(Object o) {}
+}
+
+void g1(C c) {
+  c.f('hi');
+}
+
+void g2(E e) {
+  e.f('hi');
+}
+
+main() {
+  g1(new D());
+  g2(new F());
+}