dart:html perf optimization based on runing Dromaeo benchmarks

samples/third_party/dromaeo/tests/dom-attr-htmlfast.dart

+#library("dom_attr");
+#import("dart:html");
+#import('runner.dart');
+
+void main() {
+  final int num = 10240;
+
+  // Try to force real results.
+  var ret;
+
+  Element elem = document.query('#test1');
+  Element a = document.query('a');
+
+  new Suite(window, 'dom-attr')
+    .test('getAttribute', () {
+      for (int i = 0; i < num; i++)
+        ret = elem.$dom_getAttribute('id');
+    })
+    .test('element.property', () {
+      for (int i = 0; i < num * 2; i++)
+        ret = elem.id;
+    })
+    .test('setAttribute', () {
+        for (int i = 0; i < num; i++)
+        a.$dom_setAttribute('id', 'foo');
+    })
+    .test('element.property = value', () {
+      for (int i = 0; i < num; i++)
+        a.id = 'foo';
+    })
+    .end();
+}