Add tests for is-checks on hidden native classes.

tests/language/src/NativeClassIsCheck1FrogTest.dart

+// Copyright (c) 2011, 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.
+
+// Test for correct simple is-checks on hidden native classes.
+
+interface I {
+  I read();
+  write(I x);
+}
+
+// Native implementation.
+
+class A implements I native "*A" {
+  // The native class accepts only other native instances.
+  A read() native;
+  write(A x) native;
+}
+
+makeA() native;
+
+void setup() native """
+// This code is all inside 'setup' and so not accesible from the global scope.
+function A(){}
+A.prototype.read = function() { return this._x; };
+A.prototype.write = function(x) { this._x = x; };
+""";
+
+class B {}
+
+main() {
+  setup();
+
+  var a1 = makeA();
+  var ob = new Object();
+
+  Expect.isFalse(ob is I);
+  Expect.isFalse(ob is A);
+  Expect.isFalse(ob is B);
+
+  Expect.isTrue(a1 is I);
+  Expect.isTrue(a1 is A);
+  Expect.isTrue(a1 is !B);
+}