Make getKeys, getValues getters (keys, values).

tests/html/htmlelement_test.dart

 #library('ElementTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
   test('InnerHTML', () {
     Element element = new Element.tag('div');
     element.id = 'test';
@@ skipping 51 matching lines @@
     final keys = <String> [];
     final values = <String> [];
     div.dataAttributes.forEach(void f(String key, String value) {
         keys.add(key);
         values.add(value);
     });
     Expect.setEquals(const <String> ['foo', 'bar'], keys);
     Expect.setEquals(const <String> ['foo-value', 'bar-value'], values);
 
     Expect.setEquals(const <String> ['foo', 'bar'],
-                     new List<String>.from(div.dataAttributes.getKeys()));
+                     new List<String>.from(div.dataAttributes.keys));
     Expect.setEquals(const <String> ['foo-value', 'bar-value'],
-                     new List<String>.from(div.dataAttributes.getValues()));
+                     new List<String>.from(div.dataAttributes.values));
 
     Expect.equals(2, div.dataAttributes.length);
     Expect.isFalse(div.dataAttributes.isEmpty);
 
     Expect.isNull(div.dataAttributes.remove('qux'));
     Expect.equals(2, div.dataAttributes.length);
     Expect.isFalse(div.dataAttributes.isEmpty);
 
     Expect.equals('foo-value', div.dataAttributes.remove('foo'));
     Expect.equals(1, div.dataAttributes.length);
     Expect.isFalse(div.dataAttributes.isEmpty);
 
     div.dataAttributes.clear();
     Expect.equals(0, div.dataAttributes.length);
     Expect.isTrue(div.dataAttributes.isEmpty);
   });
 }