feat(CSSClass): support binding to classList

Closes #876

modules/angular2/src/facade/collection.dart

@@ -69,6 +69,9 @@ class StringMapWrapper {
   static void set(Map map, key, value) {
     map[key] = value;
   }
+  static void delete(Map m, k) {
+      m.remove(k);
+  }
   static void forEach(Map m, fn(v, k)) {
     m.forEach((k, v) => fn(v, k));
   }

modules/angular2/src/facade/collection.es6

@@ -62,6 +62,7 @@ export class StringMapWrapper {
     }
     return true;
   }
+  static delete(map, key) { delete map[key]; }
   static forEach(map, callback) {
     for (var prop in map) {
       if (map.hasOwnProperty(prop)) {