feat(facade): do not reexport Observable from angular2/core

BREAKING CHANGE

Before import {Observable} from 'angular2/core'

After import {Observable} from 'rxjs/Observable';

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

@@ -7,7 +7,7 @@ import {Injectable} from 'angular2/core';
 import {BrowserJsonp} from './browser_jsonp';
 import {makeTypeError} from 'angular2/src/facade/exceptions';
 import {StringWrapper, isPresent} from 'angular2/src/facade/lang';
-import {Observable} from 'angular2/core';
+import {Observable} from 'rxjs/Observable';
 
 const JSONP_ERR_NO_CALLBACK = 'JSONP injected script did not invoke callback.';
 const JSONP_ERR_WRONG_METHOD = 'JSONP requests must use GET request method.';

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

@@ -7,7 +7,7 @@ import {ResponseOptions, BaseResponseOptions} from '../base_response_options';
 import {Injectable} from 'angular2/core';
 import {BrowserXhr} from './browser_xhr';
 import {isPresent} from 'angular2/src/facade/lang';
-import {Observable} from 'angular2/core';
+import {Observable} from 'rxjs/Observable';
 import {isSuccess, getResponseURL} from '../http_utils';
 /**
 * Creates connections using `XMLHttpRequest`. Given a fully-qualified

modules/angular2/src/http/http.ts

@@ -6,7 +6,7 @@ import {Request} from './static_request';
 import {Response} from './static_response';
 import {BaseRequestOptions, RequestOptions} from './base_request_options';
 import {RequestMethod} from './enums';
-import {Observable} from 'angular2/core';
+import {Observable} from 'rxjs/Observable';
 
 function httpRequest(backend: ConnectionBackend, request: Request): Observable<Response> {
   return backend.createConnection(request).response;