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/metadata/directives.ts

@@ -957,6 +957,7 @@ export class ComponentMetadata extends DirectiveMetadata {
 @CONST()
 export class PipeMetadata extends InjectableMetadata {
   name: string;
+  /** @internal */
   _pure: boolean;
 
   constructor({name, pure}: {name: string, pure: boolean}) {