chore(typings): mark underscore methods @internal.

This allows TypeScript to produce an API surface which matches the Dart semantics.
I found these with:
gulp build.js.dev && find dist/js/dev/es5/angular2/src -name "*.d.ts" -exec grep -H -n '^ *_' {} \;

Closes #4638

modules/angular2/src/core/util/decorators.ts

@@ -236,8 +236,8 @@ if (!(Reflect && Reflect.getMetadata)) {
   throw 'reflect-metadata shim is required when using class decorators';
 }
 
-export function makeDecorator(annotationCls, chainFn: (fn: Function) => void = null): (...args: any[]) =>
-    (cls: any) => any {
+export function makeDecorator(annotationCls, chainFn: (fn: Function) => void = null):
+    (...args: any[]) => (cls: any) => any {
   function DecoratorFactory(objOrType): (cls: any) => any {
     var annotationInstance = new (<any>annotationCls)(objOrType);
     if (this instanceof annotationCls) {