Check for null when invoking a native method.

frog/tests/native/src/NativeNullFrogTest.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 that parameters in native methods are not mangled. This test is needed
+// until we change all libraries to using the JS foreign element.
+
+class A native "*A" {
+  returnNull() native "return null;";
+  returnUndefined() native "return undefined;";
+  returnEmptyString() native "return '';";
+  returnZero() native "return 0;";
+}
+
+A makeA() native;
+
+void setup() native """
+function A() {}
+makeA = function(){return new A;};
+""";
+
+
+main() {
+  setup();
+  A a = makeA();
+  Expect.equals(null, a.returnNull());
+  Expect.equals(null, a.returnUndefined());
+
+  Expect.equals('', a.returnEmptyString());
+  Expect.isTrue(a.returnEmptyString().isEmpty());
+  Expect.isTrue(a.returnEmptyString() is String);
+
+  Expect.isTrue(a.returnZero() is int);
+  Expect.equals(0, a.returnZero());
+}