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diff --git a/devtools/client/responsive/utils/e10s.js b/devtools/client/responsive/utils/e10s.js
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+++ b/devtools/client/responsive/utils/e10s.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// The prefix used for RDM messages in content.
+// see: devtools/client/responsive/browser/content.js
+const MESSAGE_PREFIX = "ResponsiveMode:";
+const REQUEST_DONE_SUFFIX = ":Done";
+
+/**
+ * Registers a message `listener` that is called every time messages of
+ * specified `message` is emitted on the given message manager.
+ * @param {nsIMessageListenerManager} mm
+ *    The Message Manager
+ * @param {String} message
+ *    The message. It will be prefixed with the constant `MESSAGE_PREFIX`
+ * @param {Function} listener
+ *    The listener function that processes the message.
+ */
+function on(mm, message, listener) {
+  mm.addMessageListener(MESSAGE_PREFIX + message, listener);
+}
+exports.on = on;
+
+/**
+ * Removes a message `listener` for the specified `message` on the given
+ * message manager.
+ * @param {nsIMessageListenerManager} mm
+ *    The Message Manager
+ * @param {String} message
+ *    The message. It will be prefixed with the constant `MESSAGE_PREFIX`
+ * @param {Function} listener
+ *    The listener function that processes the message.
+ */
+function off(mm, message, listener) {
+  mm.removeMessageListener(MESSAGE_PREFIX + message, listener);
+}
+exports.off = off;
+
+/**
+ * Resolves a promise the next time the specified `message` is sent over the
+ * given message manager.
+ * @param {nsIMessageListenerManager} mm
+ *    The Message Manager
+ * @param {String} message
+ *    The message. It will be prefixed with the constant `MESSAGE_PREFIX`
+ * @returns {Promise}
+ *    A promise that is resolved when the given message is emitted.
+ */
+function once(mm, message) {
+  return new Promise(resolve => {
+    on(mm, message, function onMessage({ data }) {
+      off(mm, message, onMessage);
+      resolve(data);
+    });
+  });
+}
+exports.once = once;
+
+/**
+ * Asynchronously emit a `message` to the listeners of the given message
+ * manager.
+ *
+ * @param {nsIMessageListenerManager} mm
+ *    The Message Manager
+ * @param {String} message
+ *    The message. It will be prefixed with the constant `MESSAGE_PREFIX`.
+ * @param {Object} data
+ *    A JSON object containing data to be delivered to the listeners.
+ */
+function emit(mm, message, data) {
+  mm.sendAsyncMessage(MESSAGE_PREFIX + message, data);
+}
+exports.emit = emit;
+
+/**
+ * Asynchronously send a "request" over the given message manager, and returns
+ * a promise that is resolved when the request is complete.
+ *
+ * @param {nsIMessageListenerManager} mm
+ *    The Message Manager
+ * @param {String} message
+ *    The message. It will be prefixed with the constant `MESSAGE_PREFIX`, and
+ *    also suffixed with `REQUEST_DONE_SUFFIX` for the reply.
+ * @param {Object} data
+ *    A JSON object containing data to be delivered to the listeners.
+ * @returns {Promise}
+ *    A promise that is resolved when the request is done.
+ */
+function request(mm, message, data) {
+  const done = once(mm, message + REQUEST_DONE_SUFFIX);
+
+  emit(mm, message, data);
+
+  return done;
+}
+exports.request = request;