refactor(http): use shared code from public 'angular2/angular2' namespace

Closes #4648

modules/angular2/src/http/backends/browser_jsonp.ts

@@ -1,4 +1,4 @@
-import {Injectable} from 'angular2/src/core/di';
+import {Injectable} from 'angular2/angular2';
 import {global} from 'angular2/src/core/facade/lang';
 
 let _nextRequestId = 0;

modules/angular2/src/http/backends/browser_xhr.ts

@@ -1,4 +1,4 @@
-import {Injectable} from 'angular2/src/core/di';
+import {Injectable} from 'angular2/angular2';
 
 // Make sure not to evaluate this in a non-browser environment!
 @Injectable()

modules/angular2/src/http/backends/jsonp_backend.ts

@@ -3,7 +3,7 @@ import {ReadyStates, RequestMethods} from '../enums';
 import {Request} from '../static_request';
 import {Response} from '../static_response';
 import {ResponseOptions, BaseResponseOptions} from '../base_response_options';
-import {Injectable} from 'angular2/src/core/di';
+import {Injectable} from 'angular2/angular2';
 import {BrowserJsonp} from './browser_jsonp';
 import {EventEmitter, ObservableWrapper} from 'angular2/src/core/facade/async';
 import {makeTypeError} from 'angular2/src/core/facade/exceptions';

modules/angular2/src/http/backends/mock_backend.ts

@@ -1,4 +1,4 @@
-import {Injectable} from 'angular2/src/core/di';
+import {Injectable} from 'angular2/angular2';
 import {Request} from '../static_request';
 import {Response} from '../static_response';
 import {ReadyStates} from '../enums';

modules/angular2/src/http/backends/xhr_backend.ts

@@ -3,7 +3,7 @@ import {ReadyStates, RequestMethods, ResponseTypes} from '../enums';
 import {Request} from '../static_request';
 import {Response} from '../static_response';
 import {ResponseOptions, BaseResponseOptions} from '../base_response_options';
-import {Injectable} from 'angular2/src/core/di';
+import {Injectable} from 'angular2/angular2';
 import {BrowserXhr} from './browser_xhr';
 import {isPresent} from 'angular2/src/core/facade/lang';
 // todo(robwormald): temporary until https://github.com/angular/angular/issues/4390 decided