Add tests demonstrating bugs #30547 and #30552.

pkg/front_end/testcases/inference_new/multiple_interface_inheritance.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.
+
+/*@testedFeatures=inference*/
+library test;
+
+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. /*@target=I2::f*/ f('hi');
+}
+
+void g2(E e) {
+  e. /*@target=I2::f*/ f('hi');
+}
+
+main() {
+  g1(new D());
+  g2(new F());
+}