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diff --git a/dom/downloads/tests/clear_all_done_helper.js b/dom/downloads/tests/clear_all_done_helper.js
new file mode 100644
index 000000000..62fa1a2f3
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+++ b/dom/downloads/tests/clear_all_done_helper.js
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+/**
+ * A helper to clear out the existing downloads known to the mozDownloadManager
+ * / downloads.js.
+ *
+ * It exists because previously mozDownloadManager.clearAllDone() thought that
+ * when it returned that all the completed downloads would be cleared out.  It
+ * was wrong and this led to various intermittent test failurse.  In discussion
+ * on https://bugzil.la/979446#c13 and onwards, it was decided that
+ * clearAllDone() was in the wrong and that the jsdownloads API it depends on
+ * was not going to change to make it be in the right.
+ *
+ * The existing uses of clearAllDone() in tests seemed to be about:
+ * - Exploding if there was somehow still a download in progress
+ * - Clearing out the download list at the start of a test so that calls to
+ *   getDownloads() wouldn't have to worry about existing downloads, etc.
+ *
+ * From discussion, the right way to handle clearing is to wait for the expected
+ * removal events to occur for the existing downloads.  So that's what we do.
+ * We still generate a test failure if there are any in-progress downloads.
+ *
+ * @param {Boolean} [getDownloads=false]
+ *   If true, invoke getDownloads after clearing the download list and return
+ *   its value.
+ */
+function clearAllDoneHelper(getDownloads) {
+  var clearedPromise = new Promise(function(resolve, reject) {
+    function gotDownloads(downloads) {
+      // If there are no downloads, we're already done.
+      if (downloads.length === 0) {
+        resolve();
+        return;
+      }
+
+      // Track the set of expected downloads that will be finalized.
+      var expectedIds = new Set();
+      function changeHandler(evt) {
+        var download = evt.download;
+        if (download.state === "finalized") {
+          expectedIds.delete(download.id);
+          if (expectedIds.size === 0) {
+            resolve();
+          }
+        }
+      }
+      downloads.forEach(function(download) {
+        if (download.state === "downloading") {
+          ok(false, "A download is still active: " + download.path);
+          reject("Active download");
+        }
+        download.onstatechange = changeHandler;
+        expectedIds.add(download.id);
+      });
+      navigator.mozDownloadManager.clearAllDone();
+    }
+    function gotBadNews(err) {
+      ok(false, "Problem clearing all downloads: " + err);
+      reject(err);
+    }
+    navigator.mozDownloadManager.getDownloads().then(gotDownloads, gotBadNews);
+ });
+ if (!getDownloads) {
+   return clearedPromise;
+ }
+ return clearedPromise.then(function() {
+   return navigator.mozDownloadManager.getDownloads();
+ });
+}