Fix algorithm to know whether we should emit an is check on a class: all interfaces/implemented cla…

tests/language/is_interfaces2_test.dart

 // Copyright (c) 2012, 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 A {
 }
 
 class B extends A {
 }
 
-class C implements B {
+class C extends B {
 }
 
-int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+class D implements C {
+}
+
+int inscrutable(int x) => (x == 0) ? 0 : (x | inscrutable(x & (x - 1)));
 
 main() {
-  var things = [new A(), new B(), new C()];
+  var things = [new A(), new B(), new C(), new D()];
 
   var a = things[inscrutable(0)];
   Expect.isTrue(a is A);
   Expect.isFalse(a is B);
   Expect.isFalse(a is C);
+  Expect.isFalse(a is D);
 
   var b = things[inscrutable(1)];
   Expect.isTrue(b is A);
   Expect.isTrue(b is B);
   Expect.isFalse(b is C);
+  Expect.isFalse(b is D);
 
   var c = things[inscrutable(2)];
   Expect.isTrue(c is A);
   Expect.isTrue(c is B);
   Expect.isTrue(c is C);
+  Expect.isFalse(c is D);
+
+  var d = things[inscrutable(3)];
+  Expect.isTrue(d is A);
+  Expect.isTrue(d is B);
+  Expect.isTrue(d is C);
+  Expect.isTrue(d is D);
 }