fix(core): resurrect OnChange interface

modules/angular2/src/core/compiler/interfaces.ts

@@ -0,0 +1,7 @@
+import {StringMap} from 'angular2/src/facade/collection';
+
+/**
+ * Defines lifecycle method [onChange] called after all of component's bound
+ * properties are updated.
+ */
+export interface OnChange { onChange(changes: StringMap<string, any>): void; }