feat(dom): add replaceChild to DOM adapter

modules/angular2/src/dom/browser_adapter.dart

@@ -67,6 +67,9 @@ class BrowserDomAdapter extends GenericBrowserDomAdapter {
   void removeChild(Element el, Node node) {
     node.remove();
   }
+  void replaceChild(Node el, Node newNode, Node oldNode) {
+    oldNode.replaceWith(newNode);
+  }
   Element remove(Element el) {
     return el..remove();
   }

modules/angular2/src/dom/browser_adapter.es6

@@ -90,6 +90,9 @@ export class BrowserDomAdapter extends GenericBrowserDomAdapter {
   removeChild(el, node) {
     el.removeChild(node);
   }
+  replaceChild(el: Node, newChild, oldChild) {
+    el.replaceChild(newChild, oldChild);
+  }
   remove(el) {
     var parent = el.parentNode;
     parent.removeChild(el);

modules/angular2/src/dom/dom_adapter.js

@@ -90,6 +90,9 @@ export class DomAdapter {
   removeChild(el, node) {
     throw _abstract();
   }
+  replaceChild(el, newNode, oldNode) {
+    throw _abstract();
+  }
   remove(el) {
     throw _abstract();
   }