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diff --git a/testing/web-platform/tests/service-workers/cache-storage/resources/testharness-helpers.js b/testing/web-platform/tests/service-workers/cache-storage/resources/testharness-helpers.js
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index 000000000..e4885727b
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+++ b/testing/web-platform/tests/service-workers/cache-storage/resources/testharness-helpers.js
@@ -0,0 +1,33 @@
+/*
+ * testharness-helpers contains various useful extensions to testharness.js to
+ * allow them to be used across multiple tests before they have been
+ * upstreamed. This file is intended to be usable from both document and worker
+ * environments, so code should for example not rely on the DOM.
+ */
+
+// Returns a promise that fulfills after the provided |promise| is fulfilled.
+// The |test| succeeds only if |promise| rejects with an exception matching
+// |code|. Accepted values for |code| follow those accepted for assert_throws().
+// The optional |description| describes the test being performed.
+//
+// E.g.:
+//   assert_promise_rejects(
+//       new Promise(...), // something that should throw an exception.
+//       'NotFoundError',
+//       'Should throw NotFoundError.');
+//
+//   assert_promise_rejects(
+//       new Promise(...),
+//       new TypeError(),
+//       'Should throw TypeError');
+function assert_promise_rejects(promise, code, description) {
+  return promise.then(
+    function() {
+      throw 'assert_promise_rejects: ' + description + ' Promise did not reject.';
+    },
+    function(e) {
+      if (code !== undefined) {
+        assert_throws(code, function() { throw e; }, description);
+      }
+    });
+}