feat(http): refactor library to work in dart

Mostly internal refactoring needed to make ts2dart and DartAnalyzer happy.

Fixes #2415

modules/angular2/src/http/enums.ts

@@ -1,3 +1,5 @@
+import {StringMap, StringMapWrapper} from 'angular2/src/facade/collection';
+
 export enum RequestModesOpts {
   Cors,
   NoCors,
@@ -29,6 +31,14 @@ export enum RequestMethods {
   PATCH
 }
 
+// TODO: Remove this when enum lookups are available in ts2dart
+// https://github.com/angular/ts2dart/issues/221
+export class RequestMethodsMap {
+  private _methods: List<string>;
+  constructor() { this._methods = ['GET', 'POST', 'PUT', 'DELETE', 'OPTIONS', 'HEAD', 'PATCH']; }
+  getMethod(method: int): string { return this._methods[method]; }
+}
+
 export enum ReadyStates {
   UNSENT,
   OPEN,