feat(change_detection): implement a change detector generator

modules/facade/src/collection.dart

@@ -89,8 +89,8 @@ class ListWrapper {
   static reduce(List list, Function fn, init) {
     return list.fold(init, fn);
   }
-  static first(List list) => list.first;
-  static last(List list) => list.last;
+  static first(List list) => list.isEmpty ? null : list.first;
+  static last(List list) => list.isEmpty ? null : list.last;
   static List reversed(List list) => list.reversed.toList();
   static void push(List l, e) { l.add(e); }
   static List concat(List a, List b) {a.addAll(b); return a;}

modules/facade/src/lang.dart

@@ -27,6 +27,7 @@ class IMPLEMENTS {
 
 bool isPresent(obj) => obj != null;
 bool isBlank(obj) => obj == null;
+bool isString(obj) => obj is String;
 
 String stringify(obj) => obj.toString();
 

modules/facade/src/lang.es6

@@ -27,6 +27,10 @@ export function isBlank(obj):boolean {
   return obj === undefined || obj === null;
 }
 
+export function isString(obj):boolean {
+  return typeof obj === "string";
+}
+
 export function stringify(token):string {
   if (typeof token === 'string') {
     return token;