fix(XHRImpl): fix errors, add a spec

fixes #1715

modules/angular2/test/services/static_assets/200.html

@@ -0,0 +1 @@
+<p>hey</p>

modules/angular2/test/services/xhr_impl_spec.js

@@ -0,0 +1,43 @@
+import {
+    AsyncTestCompleter,
+    beforeEach,
+    ddescribe,
+    describe,
+    expect,
+    iit,
+    inject,
+    it,
+    xit
+} from 'angular2/test_lib';
+
+import {XHRImpl} from 'angular2/src/services/xhr_impl';
+import {PromiseWrapper} from 'angular2/src/facade/async';
+
+export function main() {
+  describe('XHRImpl', () => {
+    var xhr;
+    var url200 = '/base/modules/angular2/test/services/static_assets/200.html';
+    var url404 = '/base/modules/angular2/test/services/static_assets/404.html';
+
+    beforeEach(() => {
+      xhr = new XHRImpl();
+    });
+
+    it('should resolve the Promise with the file content on success', inject([AsyncTestCompleter], (async) => {
+      xhr.get(url200).then((text) => {
+        expect(text.trim()).toEqual('<p>hey</p>');
+        async.done();
+      });
+    }));
+
+    it('should reject the Promise on failure', inject([AsyncTestCompleter], (async) => {
+      PromiseWrapper.catchError(
+          xhr.get(url404),
+          (e) => {
+            expect(e).toEqual(`Failed to load ${url404}`);
+            async.done();
+          }
+      );
+    }));
+  });
+}