feat(test_lib): add method to compare stringified DOM element

Closes #2106

modules/angular2/src/facade/collection.dart

@@ -172,8 +172,12 @@ class ListWrapper {
     l.removeRange(from, to);
     return sub;
   }
-  static void sort(List l, compareFn(a, b)) {
-    l.sort(compareFn);
+  static void sort(List l, [compareFn(a, b) = null]) {
+    if (compareFn == null) {
+      l.sort();
+    } else {
+      l.sort(compareFn);
+    }
   }
 
   // JS splice, slice, fill functions can take start < 0 which indicates a position relative to

modules/angular2/src/facade/collection.ts

@@ -1,4 +1,4 @@
-import {isJsObject, global} from 'angular2/src/facade/lang';
+import {isJsObject, global, isPresent} from 'angular2/src/facade/lang';
 
 export var List = global.Array;
 export var Map = global.Map;
@@ -231,7 +231,13 @@ export class ListWrapper {
     return l.slice(from, to === null ? undefined : to);
   }
   static splice<T>(l: List<T>, from: int, length: int): List<T> { return l.splice(from, length); }
-  static sort<T>(l: List<T>, compareFn: (a: T, b: T) => number) { l.sort(compareFn); }
+  static sort<T>(l: List<T>, compareFn?: (a: T, b: T) => number) {
+    if (isPresent(compareFn)) {
+      l.sort(compareFn);
+    } else {
+      l.sort();
+    }
+  }
 }
 
 export function isListLikeIterable(obj): boolean {