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/http/headers.ts

@@ -35,6 +35,7 @@ import {
  * ```
  */
 export class Headers {
+  /** @internal */
   _headersMap: Map<string, string[]>;
   constructor(headers?: Headers | {[key: string]: any}) {
     if (headers instanceof Headers) {