Add a .text() accessor to FetchResponse (#2922)

Add a .text() accessor to FetchResponse to allow retrieving text without converting it to JSON first.  (I.e., extending FetchResponse to cover one additional method from the Response / Body API.)

src/xhr.js

@@ -357,7 +357,7 @@ function assertSuccess(response) {
  *
  * See https://developer.mozilla.org/en-US/docs/Web/API/GlobalFetch/fetch
  */
-class FetchResponse {
+export class FetchResponse {
   /**
    * @param {!XMLHttpRequest} xhr
    */
@@ -386,6 +386,15 @@ class FetchResponse {
     return Promise.resolve(this.xhr_.responseText);
   }
 
+  /**
+   * Drains the response and returns a promise that resolves with the response
+   * text.
+   * @return {!Promise<string>}
+   */
+  text() {
+    return this.drainText_();
+  }
+
   /**
    * Drains the response and returns the JSON object.
    * @return {!Promise<!JSONValue>}

test/functional/test-xhr.js

@@ -15,7 +15,7 @@
  */
 
 import * as sinon from 'sinon';
-import {xhrFor, fetchPolyfill} from '../../src/xhr';
+import {xhrFor, fetchPolyfill, FetchResponse} from '../../src/xhr';
 
 describe('XHR', function() {
   let sandbox;
@@ -413,4 +413,51 @@ describe('XHR', function() {
 
     });
   });
+
+  describe('FetchResponse', () => {
+    const TEST_TEXT = 'this is some test text';
+    const mockXhr = {
+      status: 200,
+      responseText: TEST_TEXT,
+    };
+
+    it('should provide text', () => {
+      const response = new FetchResponse(mockXhr);
+      return response.text().then(result => {
+        expect(result).to.equal(TEST_TEXT);
+      });
+    });
+
+    it('should provide text only once', () => {
+      const response = new FetchResponse(mockXhr);
+      return response.text().then(result => {
+        expect(result).to.equal(TEST_TEXT);
+        expect(response.text.bind(response), 'should throw').to.throw(Error,
+            /Body already used/);
+      });
+    });
+
+    scenarios.forEach(test => {
+      if (test.desc === 'Polyfill') {
+        // FetchRequest is only returned by the Polyfill version of Xhr.
+        describe('#text', () => {
+          beforeEach(setupMockXhr);
+          it('should return text from a full XHR request', () => {
+            expect(requests[0]).to.be.undefined;
+            const promise = test.xhr.fetchAmpCors_('http://nowhere.org').then(
+                response => {
+                  expect(response).to.be.instanceof(FetchResponse);
+                  return response.text().then(result => {
+                    expect(result).to.equal(TEST_TEXT);
+                  });
+                });
+            requests[0].respond(200, {
+              'Content-Type': 'text/plain',
+            }, TEST_TEXT);
+            return promise;
+          });
+        });
+      }
+    });
+  });
 });