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diff --git a/remote/marionette/stream-utils.sys.mjs b/remote/marionette/stream-utils.sys.mjs
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+++ b/remote/marionette/stream-utils.sys.mjs
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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/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  EventEmitter: "resource://gre/modules/EventEmitter.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "IOUtil",
+  "@mozilla.org/io-util;1",
+  "nsIIOUtil"
+);
+
+ChromeUtils.defineLazyGetter(lazy, "ScriptableInputStream", () => {
+  return Components.Constructor(
+    "@mozilla.org/scriptableinputstream;1",
+    "nsIScriptableInputStream",
+    "init"
+  );
+});
+
+const BUFFER_SIZE = 0x8000;
+
+/**
+ * This helper function (and its companion object) are used by bulk
+ * senders and receivers to read and write data in and out of other streams.
+ * Functions that make use of this tool are passed to callers when it is
+ * time to read or write bulk data.  It is highly recommended to use these
+ * copier functions instead of the stream directly because the copier
+ * enforces the agreed upon length. Since bulk mode reuses an existing
+ * stream, the sender and receiver must write and read exactly the agreed
+ * upon amount of data, or else the entire transport will be left in a
+ * invalid state.  Additionally, other methods of stream copying (such as
+ * NetUtil.asyncCopy) close the streams involved, which would terminate
+ * the debugging transport, and so it is avoided here.
+ *
+ * Overall, this *works*, but clearly the optimal solution would be
+ * able to just use the streams directly.  If it were possible to fully
+ * implement nsIInputStream/nsIOutputStream in JS, wrapper streams could
+ * be created to enforce the length and avoid closing, and consumers could
+ * use familiar stream utilities like NetUtil.asyncCopy.
+ *
+ * The function takes two async streams and copies a precise number
+ * of bytes from one to the other.  Copying begins immediately, but may
+ * complete at some future time depending on data size.  Use the returned
+ * promise to know when it's complete.
+ *
+ * @param {nsIAsyncInputStream} input
+ *     Stream to copy from.
+ * @param {nsIAsyncOutputStream} output
+ *        Stream to copy to.
+ * @param {number} length
+ *        Amount of data that needs to be copied.
+ *
+ * @returns {Promise}
+ *     Promise is resolved when copying completes or rejected if any
+ *     (unexpected) errors occur.
+ */
+function copyStream(input, output, length) {
+  let copier = new StreamCopier(input, output, length);
+  return copier.copy();
+}
+
+/** @class */
+function StreamCopier(input, output, length) {
+  lazy.EventEmitter.decorate(this);
+  this._id = StreamCopier._nextId++;
+  this.input = input;
+  // Save off the base output stream, since we know it's async as we've
+  // required
+  this.baseAsyncOutput = output;
+  if (lazy.IOUtil.outputStreamIsBuffered(output)) {
+    this.output = output;
+  } else {
+    this.output = Cc[
+      "@mozilla.org/network/buffered-output-stream;1"
+    ].createInstance(Ci.nsIBufferedOutputStream);
+    this.output.init(output, BUFFER_SIZE);
+  }
+  this._length = length;
+  this._amountLeft = length;
+  this._deferred = {
+    promise: new Promise((resolve, reject) => {
+      this._deferred.resolve = resolve;
+      this._deferred.reject = reject;
+    }),
+  };
+
+  this._copy = this._copy.bind(this);
+  this._flush = this._flush.bind(this);
+  this._destroy = this._destroy.bind(this);
+
+  // Copy promise's then method up to this object.
+  //
+  // Allows the copier to offer a promise interface for the simple succeed
+  // or fail scenarios, but also emit events (due to the EventEmitter)
+  // for other states, like progress.
+  this.then = this._deferred.promise.then.bind(this._deferred.promise);
+  this.then(this._destroy, this._destroy);
+
+  // Stream ready callback starts as |_copy|, but may switch to |_flush|
+  // at end if flushing would block the output stream.
+  this._streamReadyCallback = this._copy;
+}
+StreamCopier._nextId = 0;
+
+StreamCopier.prototype = {
+  copy() {
+    // Dispatch to the next tick so that it's possible to attach a progress
+    // event listener, even for extremely fast copies (like when testing).
+    Services.tm.currentThread.dispatch(() => {
+      try {
+        this._copy();
+      } catch (e) {
+        this._deferred.reject(e);
+      }
+    }, 0);
+    return this;
+  },
+
+  _copy() {
+    let bytesAvailable = this.input.available();
+    let amountToCopy = Math.min(bytesAvailable, this._amountLeft);
+    this._debug("Trying to copy: " + amountToCopy);
+
+    let bytesCopied;
+    try {
+      bytesCopied = this.output.writeFrom(this.input, amountToCopy);
+    } catch (e) {
+      if (e.result == Cr.NS_BASE_STREAM_WOULD_BLOCK) {
+        this._debug("Base stream would block, will retry");
+        this._debug("Waiting for output stream");
+        this.baseAsyncOutput.asyncWait(this, 0, 0, Services.tm.currentThread);
+        return;
+      }
+      throw e;
+    }
+
+    this._amountLeft -= bytesCopied;
+    this._debug("Copied: " + bytesCopied + ", Left: " + this._amountLeft);
+    this._emitProgress();
+
+    if (this._amountLeft === 0) {
+      this._debug("Copy done!");
+      this._flush();
+      return;
+    }
+
+    this._debug("Waiting for input stream");
+    this.input.asyncWait(this, 0, 0, Services.tm.currentThread);
+  },
+
+  _emitProgress() {
+    this.emit("progress", {
+      bytesSent: this._length - this._amountLeft,
+      totalBytes: this._length,
+    });
+  },
+
+  _flush() {
+    try {
+      this.output.flush();
+    } catch (e) {
+      if (
+        e.result == Cr.NS_BASE_STREAM_WOULD_BLOCK ||
+        e.result == Cr.NS_ERROR_FAILURE
+      ) {
+        this._debug("Flush would block, will retry");
+        this._streamReadyCallback = this._flush;
+        this._debug("Waiting for output stream");
+        this.baseAsyncOutput.asyncWait(this, 0, 0, Services.tm.currentThread);
+        return;
+      }
+      throw e;
+    }
+    this._deferred.resolve();
+  },
+
+  _destroy() {
+    this._destroy = null;
+    this._copy = null;
+    this._flush = null;
+    this.input = null;
+    this.output = null;
+  },
+
+  // nsIInputStreamCallback
+  onInputStreamReady() {
+    this._streamReadyCallback();
+  },
+
+  // nsIOutputStreamCallback
+  onOutputStreamReady() {
+    this._streamReadyCallback();
+  },
+
+  _debug() {},
+};
+
+/**
+ * Read from a stream, one byte at a time, up to the next
+ * <var>delimiter</var> character, but stopping if we've read |count|
+ * without finding it.  Reading also terminates early if there are less
+ * than <var>count</var> bytes available on the stream.  In that case,
+ * we only read as many bytes as the stream currently has to offer.
+ *
+ * @param {nsIInputStream} stream
+ *     Input stream to read from.
+ * @param {string} delimiter
+ *     Character we're trying to find.
+ * @param {number} count
+ *     Max number of characters to read while searching.
+ *
+ * @returns {string}
+ *     Collected data.  If the delimiter was found, this string will
+ *     end with it.
+ */
+// TODO: This implementation could be removed if bug 984651 is fixed,
+// which provides a native version of the same idea.
+function delimitedRead(stream, delimiter, count) {
+  let scriptableStream;
+  if (stream instanceof Ci.nsIScriptableInputStream) {
+    scriptableStream = stream;
+  } else {
+    scriptableStream = new lazy.ScriptableInputStream(stream);
+  }
+
+  let data = "";
+
+  // Don't exceed what's available on the stream
+  count = Math.min(count, stream.available());
+
+  if (count <= 0) {
+    return data;
+  }
+
+  let char;
+  while (char !== delimiter && count > 0) {
+    char = scriptableStream.readBytes(1);
+    count--;
+    data += char;
+  }
+
+  return data;
+}
+
+export const StreamUtils = {
+  copyStream,
+  delimitedRead,
+};