diff --git a/src/core/worker.js b/src/core/worker.js
index ad90d8ab5..4312c3856 100644
--- a/src/core/worker.js
+++ b/src/core/worker.js
@@ -14,15 +14,16 @@
  */
 
 import {
-  arrayByteLength, arraysToBytes, assert, createPromiseCapability,
-  getVerbosityLevel, info, InvalidPDFException, MissingPDFException,
-  PasswordException, setVerbosityLevel, UnexpectedResponseException,
-  UnknownErrorException, UNSUPPORTED_FEATURES, VerbosityLevel, warn
+  arrayByteLength, arraysToBytes, createPromiseCapability, getVerbosityLevel,
+  info, InvalidPDFException, MissingPDFException, PasswordException,
+  setVerbosityLevel, UnexpectedResponseException, UnknownErrorException,
+  UNSUPPORTED_FEATURES, VerbosityLevel, warn
 } from '../shared/util';
 import { clearPrimitiveCaches, Ref } from './primitives';
 import { LocalPdfManager, NetworkPdfManager } from './pdf_manager';
 import isNodeJS from '../shared/is_node';
 import { MessageHandler } from '../shared/message_handler';
+import { PDFWorkerStream } from './worker_stream';
 import { XRefParseException } from './core_utils';
 
 var WorkerTask = (function WorkerTaskClosure() {
@@ -55,122 +56,6 @@ var WorkerTask = (function WorkerTaskClosure() {
   return WorkerTask;
 })();
 
-/** @implements {IPDFStream} */
-var PDFWorkerStream = (function PDFWorkerStreamClosure() {
-  function PDFWorkerStream(msgHandler) {
-    this._msgHandler = msgHandler;
-    this._contentLength = null;
-    this._fullRequestReader = null;
-    this._rangeRequestReaders = [];
-  }
-  PDFWorkerStream.prototype = {
-    getFullReader() {
-      assert(!this._fullRequestReader);
-      this._fullRequestReader = new PDFWorkerStreamReader(this._msgHandler);
-      return this._fullRequestReader;
-    },
-
-    getRangeReader(begin, end) {
-      let reader = new PDFWorkerStreamRangeReader(begin, end, this._msgHandler);
-      this._rangeRequestReaders.push(reader);
-      return reader;
-    },
-
-    cancelAllRequests(reason) {
-      if (this._fullRequestReader) {
-        this._fullRequestReader.cancel(reason);
-      }
-      let readers = this._rangeRequestReaders.slice(0);
-      readers.forEach(function (reader) {
-        reader.cancel(reason);
-      });
-    },
-  };
-
-  /** @implements {IPDFStreamReader} */
-  function PDFWorkerStreamReader(msgHandler) {
-    this._msgHandler = msgHandler;
-
-    this._contentLength = null;
-    this._isRangeSupported = false;
-    this._isStreamingSupported = false;
-
-    let readableStream = this._msgHandler.sendWithStream('GetReader');
-
-    this._reader = readableStream.getReader();
-
-    this._headersReady = this._msgHandler.sendWithPromise('ReaderHeadersReady').
-        then((data) => {
-      this._isStreamingSupported = data.isStreamingSupported;
-      this._isRangeSupported = data.isRangeSupported;
-      this._contentLength = data.contentLength;
-    });
-  }
-  PDFWorkerStreamReader.prototype = {
-    get headersReady() {
-      return this._headersReady;
-    },
-
-    get contentLength() {
-      return this._contentLength;
-    },
-
-    get isStreamingSupported() {
-      return this._isStreamingSupported;
-    },
-
-    get isRangeSupported() {
-      return this._isRangeSupported;
-    },
-
-    read() {
-      return this._reader.read().then(function({ value, done, }) {
-        if (done) {
-          return { value: undefined, done: true, };
-        }
-        // `value` is wrapped into Uint8Array, we need to
-        // unwrap it to ArrayBuffer for further processing.
-        return { value: value.buffer, done: false, };
-      });
-    },
-
-    cancel(reason) {
-      this._reader.cancel(reason);
-    },
-  };
-
-  /** @implements {IPDFStreamRangeReader} */
-  function PDFWorkerStreamRangeReader(begin, end, msgHandler) {
-    this._msgHandler = msgHandler;
-    this.onProgress = null;
-
-    let readableStream = this._msgHandler.sendWithStream('GetRangeReader',
-                                                         { begin, end, });
-
-    this._reader = readableStream.getReader();
-  }
-  PDFWorkerStreamRangeReader.prototype = {
-    get isStreamingSupported() {
-      return false;
-    },
-
-    read() {
-      return this._reader.read().then(function({ value, done, }) {
-        if (done) {
-          return { value: undefined, done: true, };
-        }
-        return { value: value.buffer, done: false, };
-      });
-    },
-
-    cancel(reason) {
-      this._reader.cancel(reason);
-    },
-  };
-
-  return PDFWorkerStream;
-})();
-
 var WorkerMessageHandler = {
   setup(handler, port) {
     var testMessageProcessed = false;
diff --git a/src/core/worker_stream.js b/src/core/worker_stream.js
new file mode 100644
index 000000000..12c407e4b
--- /dev/null
+++ b/src/core/worker_stream.js
@@ -0,0 +1,133 @@
+/* Copyright 2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/* eslint no-var: error */
+
+import { assert } from '../shared/util';
+
+/** @implements {IPDFStream} */
+class PDFWorkerStream {
+  constructor(msgHandler) {
+    this._msgHandler = msgHandler;
+    this._contentLength = null;
+    this._fullRequestReader = null;
+    this._rangeRequestReaders = [];
+  }
+
+  getFullReader() {
+    assert(!this._fullRequestReader);
+    this._fullRequestReader = new PDFWorkerStreamReader(this._msgHandler);
+    return this._fullRequestReader;
+  }
+
+  getRangeReader(begin, end) {
+    const reader = new PDFWorkerStreamRangeReader(begin, end, this._msgHandler);
+    this._rangeRequestReaders.push(reader);
+    return reader;
+  }
+
+  cancelAllRequests(reason) {
+    if (this._fullRequestReader) {
+      this._fullRequestReader.cancel(reason);
+    }
+    const readers = this._rangeRequestReaders.slice(0);
+    readers.forEach(function(reader) {
+      reader.cancel(reason);
+    });
+  }
+}
+
+/** @implements {IPDFStreamReader} */
+class PDFWorkerStreamReader {
+  constructor(msgHandler) {
+    this._msgHandler = msgHandler;
+    this.onProgress = null;
+
+    this._contentLength = null;
+    this._isRangeSupported = false;
+    this._isStreamingSupported = false;
+
+    const readableStream = this._msgHandler.sendWithStream('GetReader');
+    this._reader = readableStream.getReader();
+
+    this._headersReady = this._msgHandler.sendWithPromise('ReaderHeadersReady').
+        then((data) => {
+      this._isStreamingSupported = data.isStreamingSupported;
+      this._isRangeSupported = data.isRangeSupported;
+      this._contentLength = data.contentLength;
+    });
+  }
+
+  get headersReady() {
+    return this._headersReady;
+  }
+
+  get contentLength() {
+    return this._contentLength;
+  }
+
+  get isStreamingSupported() {
+    return this._isStreamingSupported;
+  }
+
+  get isRangeSupported() {
+    return this._isRangeSupported;
+  }
+
+  async read() {
+    const { value, done, } = await this._reader.read();
+    if (done) {
+      return { value: undefined, done: true, };
+    }
+    // `value` is wrapped into Uint8Array, we need to
+    // unwrap it to ArrayBuffer for further processing.
+    return { value: value.buffer, done: false, };
+  }
+
+  cancel(reason) {
+    this._reader.cancel(reason);
+  }
+}
+
+/** @implements {IPDFStreamRangeReader} */
+class PDFWorkerStreamRangeReader {
+  constructor(begin, end, msgHandler) {
+    this._msgHandler = msgHandler;
+    this.onProgress = null;
+
+    const readableStream = this._msgHandler.sendWithStream('GetRangeReader',
+                                                           { begin, end, });
+    this._reader = readableStream.getReader();
+  }
+
+  get isStreamingSupported() {
+    return false;
+  }
+
+  async read() {
+    const { value, done, } = await this._reader.read();
+    if (done) {
+      return { value: undefined, done: true, };
+    }
+    return { value: value.buffer, done: false, };
+  }
+
+  cancel(reason) {
+    this._reader.cancel(reason);
+  }
+}
+
+export {
+  PDFWorkerStream,
+};
diff --git a/src/interfaces.js b/src/interfaces.js
index 66879a532..e83991685 100644
--- a/src/interfaces.js
+++ b/src/interfaces.js
@@ -67,7 +67,7 @@ class IPDFStreamReader {
    * @returns {Promise}
    */
   get headersReady() {
-    return null;
+    return Promise.resolve();
   }
 
   /**
@@ -116,7 +116,7 @@ class IPDFStreamReader {
    * set to true.
    * @returns {Promise}
    */
-  read() {}
+  async read() {}
 
   /**
    * Cancels all pending read requests and closes the stream.
@@ -157,7 +157,7 @@ class IPDFStreamRangeReader {
    * set to true.
    * @returns {Promise}
    */
-  read() {}
+  async read() {}
 
   /**
    * Cancels all pending read requests and closes the stream.