Implement is-Operator for interfaces.

frog/tests/leg/src/ResolverTest.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.
 
 #import("../../../leg/leg.dart");
 #import("../../../leg/elements/elements.dart");
 #import("../../../leg/tree/tree.dart");
 #import("../../../leg/util/util.dart");
 #import("mock_compiler.dart");
 #import("parser_helper.dart");
@@ skipping 462 matching lines @@
                           class B extends A {}
                           main() { return new A(); }""");
   FunctionElement mainElement = compiler.mainApp.find(MAIN);
   compiler.resolver.resolve(mainElement);
   Expect.equals(0, compiler.warnings.length);
   Expect.equals(1, compiler.errors.length);
   Expect.equals(MessageKind.CYCLIC_CLASS_HIERARCHY,
                 compiler.errors[0].message.kind);
 
   compiler = new MockCompiler();
+  compiler.parseScript("""interface A extends B {}
+                          interface B extends A {}
+                          class C implements A {}
+                          main() { return new C(); }""");
+  mainElement = compiler.mainApp.find(MAIN);
+  compiler.resolver.resolve(mainElement);
+  Expect.equals(0, compiler.warnings.length);
+  Expect.equals(1, compiler.errors.length);
+  Expect.equals(MessageKind.CYCLIC_CLASS_HIERARCHY,
+                compiler.errors[0].message.kind);
+
+  compiler = new MockCompiler();
   compiler.parseScript("""class A extends B {}
                           class B extends C {}
                           class C {}
                           main() { return new A(); }""");
   mainElement = compiler.mainApp.find(MAIN);
   compiler.resolver.resolve(mainElement);
   Expect.equals(0, compiler.warnings.length);
   Expect.equals(0, compiler.errors.length);
   ClassElement aElement = compiler.mainApp.find(buildSourceString("A"));
   Link<Type> supertypes = aElement.allSupertypes;
@@ skipping 95 matching lines @@
 length(Link link) => link.isEmpty() ? 0 : length(link.tail) + 1;
 
 at(Link link, int index) => (index == 0) ? link.head : at(link.tail, index - 1);
 
 List<String> asSortedStrings(Link link) {
   List<String> result = <String>[];
   for (; !link.isEmpty(); link = link.tail) result.add(link.head.toString());
   result.sort((s1, s2) => s1.compareTo(s2));
   return result;
 }