Make isEmpty a getter.

tests/compiler/dart2js_foreign/native_null_test.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.
 
 @native("*A")
 class A {
   @native("return null;")
@@ skipping 15 matching lines @@
 void setup();
 
 
 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().isEmpty);
   Expect.isTrue(a.returnEmptyString() is String);
 
   Expect.isTrue(a.returnZero() is int);
   Expect.equals(0, a.returnZero());
 }