feat(viewPort): adds initial implementation of ViewPort.

ViewPort is the mechanism backing @Template directives. Those
directives can use the viewport to dynamically create, attach and
detach views.

modules/facade/src/dom.dart

@@ -47,6 +47,12 @@ class DOM {
   static appendChild(el, node) {
     el.append(node);
   }
+  static removeChild(el, node) {
+    node.remove();
+  }
+  static insertAfter(el, node) {
+    el.parentNode.insertBefore(node, el.nextNode);
+  }
   static setText(Text text, String value) {
     text.text = value;
   }

modules/facade/src/dom.es6

@@ -40,6 +40,12 @@ export class DOM {
   static appendChild(el, node) {
     el.appendChild(node);
   }
+  static removeChild(el, node) {
+    el.removeChild(node);
+  }
+  static insertAfter(el, node) {
+    el.parentNode.insertBefore(node, el.nextSibling);
+  }
   static setInnerHTML(el, value) {
     el.innerHTML = value;
   }