Make isEmpty a getter.

tests/compiler/dart2js_native/native_null_frog_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.
 
 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().isEmpty);
   Expect.isTrue(a.returnEmptyString() is String);
 
   Expect.isTrue(a.returnZero() is int);
   Expect.equals(0, a.returnZero());
 }