Add tests demonstrating bugs #30547 and #30552.

tests/language_2/multiple_interface_inheritance_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 I1 {

[Paul Berry, 2017/08/26 14:53:44]

Whoops, I forgot to mark I1 and I2 as "abstract".  Will fix before committing.

+  void f(int i);
+}
+
+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 {}

[Jennifer Messerly, 2017/08/26 00:24:16]

oh wow -- so the analyzer analysis depends on the order here? "C" works but "E"
does not?

[Paul Berry, 2017/08/26 14:53:44]

Actually, it's the other way round.  E works but C does not.  It appears that
analyzer just resolves the call to f to whichever interface is listed first.

+
+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());
+}