dart:html perf optimization based on runing Dromaeo benchmarks

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

+#library("dom_modify_html");
+#import("dart:html");
+#import("dart:json");
+#source("Common.dart");
+#source("RunnerSuite.dart");
+
+void main() {
+  final int num = 400;
+
+  String str = 'null';
+  // Very ugly way to build up the string, but let's mimic JS version as much as possible.
+  for (int i = 0; i < 1024; i++) {
+    str += new String.fromCharCodes([((25 * Math.random()) + 97).toInt()]);
+  }
+
+  List<Node> elems = <Node>[];
+
+  // Try to force real results.
+  var ret;
+
+  final htmlstr = document.body.innerHTML;
+
+  new Suite(window, 'dom-modify')
+    .test('createElement', () {
+      for (int i = 0; i < num; i++) {
+        ret = new Element.tag('div');
+        ret = new Element.tag('span');
+        ret = new Element.tag('table');
+        ret = new Element.tag('tr');
+        ret = new Element.tag('select');
+      }
+    })
+    .test('createTextNode', () {
+      for (int i = 0; i < num; i++) {
+        ret = new Text(str);
+        ret = new Text('${str}2');
+        ret = new Text('${str}3');
+        ret = new Text('${str}4');
+        ret = new Text('${str}5');
+      }
+    })
+    .test('innerHTML', () {
+      document.body.innerHTML = htmlstr;
+    })
+    .prep(() {
+      elems = new List<Node>();
+      final telems = document.body.nodes;
+      for (int i = 0; i < telems.length; i++) {
+        elems.add(telems[i]);
+      }
+    })
+    .test('cloneNode', () {
+      for (int i = 0; i < elems.length; i++) {
+        ret = elems[i].clone(false);
+        ret = elems[i].clone(true);
+        ret = elems[i].clone(true);
+        }
+    })
+    .test('appendChild', () {
+      for (int i = 0; i < elems.length; i++)
+        document.body.$dom_appendChild(elems[i]);
+    })
+    .test('insertBefore', () {
+      for (int i = 0; i < elems.length; i++)
+        document.body.insertBefore(elems[i], document.body.$dom_firstChild);
+    })
+    .end();
+}