Bug 1153289, add some tab switching methods to BrowserTestUtils.jsm, r=felipe

testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm

@@ -51,15 +51,76 @@ this.BrowserTestUtils = {
    * @rejects Any exception from taskFn is propagated.
    */
   withNewTab: Task.async(function* (options, taskFn) {
-    let tab = options.gBrowser.addTab(options.url);
-    yield BrowserTestUtils.browserLoaded(tab.linkedBrowser);
-    options.gBrowser.selectedTab = tab;
-
+    let tab = yield BrowserTestUtils.openNewForegroundTab(options.gBrowser, options.url);
     yield taskFn(tab.linkedBrowser);
-
     options.gBrowser.removeTab(tab);
   }),
 
+  /**
+   * Opens a new tab in the foreground.
+   *
+   * @param {tabbrowser} tabbrowser
+   *        The tabbrowser to open the tab new in.
+   * @param {string} opening
+   *        May be either a string URL to load in the tab, or a function that
+   *        will be called to open a foreground tab. Defaults to "about:blank".
+   * @param {boolean} waitForLoad
+   *        True to wait for the page in the new tab to load. Defaults to true.
+   *
+   * @return {Promise}
+   *         Resolves when the tab is ready and loaded as necessary.
+   * @resolves The new tab.
+   */
+  openNewForegroundTab(tabbrowser, opening = "about:blank", aWaitForLoad = true) {
+    let tab;
+    let promises = [
+      BrowserTestUtils.switchTab(tabbrowser, function () {
+        if (typeof opening == "function") {
+          opening();
+          tab = tabbrowser.selectedTab;
+        }
+        else {
+          tabbrowser.selectedTab = tab = tabbrowser.addTab(opening);
+        }
+      })
+    ];
+
+    if (aWaitForLoad) {
+      promises.push(BrowserTestUtils.browserLoaded(tab.linkedBrowser));
+    }
+
+    return Promise.all(promises).then(() => tab);
+  },
+
+  /**
+   * Switches to a tab and resolves when it is ready.
+   *
+   * @param {tabbrowser} tabbrowser
+   *        The tabbrowser.
+   * @param {tab} tab
+   *        Either a tab element to switch to or a function to perform the switch.
+   *
+   * @return {Promise}
+   *         Resolves when the tab has been switched to.
+   * @resolves The tab switched to.
+   */
+  switchTab(tabbrowser, tab) {
+    let promise = new Promise(resolve => {
+      tabbrowser.addEventListener("TabSwitchDone", function onSwitch() {
+        tabbrowser.removeEventListener("TabSwitchDone", onSwitch);
+        TestUtils.executeSoon(() => resolve(tabbrowser.selectedTab));
+      });
+    });
+
+    if (typeof tab == "function") {
+      tab();
+    }
+    else {
+      tabbrowser.selectedTab = tab;
+    }
+    return promise;
+  },
+
   /**
    * Waits for an ongoing page load in a browser window to complete.
    *