feat(i18n): implement xmb serializer

modules/angular2/src/i18n/xmb_serializer.ts

@@ -0,0 +1,12 @@
+import {isPresent} from 'angular2/src/facade/lang';
+import {Message, id} from './message';
+
+export function serialize(messages: Message[]): string {
+  let ms = messages.map((m) => _serializeMessage(m)).join("");
+  return `<message-bundle>${ms}</message-bundle>`;
+}
+
+function _serializeMessage(m: Message): string {
+  let desc = isPresent(m.description) ? ` desc='${m.description}'` : "";
+  return `<msg id='${id(m)}'${desc}>${m.content}</msg>`;
+}