perf: improve baseline speed by 30%

Use node.firstChild and node.nextSibling instead 
of node.children or node.childNodes in the baseline
benchmark.

modules/facade/src/dom.dart

@@ -38,6 +38,9 @@ class DOM {
   static Node firstChild(el) {
     return el.firstChild;
   }
+  static Node nextSibling(el) {
+    return el.nextNode;
+  }
   static Element parentElement(el) {
     return el.parent;
   }

modules/facade/src/dom.es6

@@ -31,6 +31,9 @@ export class DOM {
   static firstChild(el):Node {
     return el.firstChild;
   }
+  static nextSibling(el):Node {
+    return el.nextSibling;
+  }
   static parentElement(el) {
     return el.parentElement;
   }