diff --git a/build/build.py b/build/build.py
index b32e097348..839672cc05 100755
--- a/build/build.py
+++ b/build/build.py
@@ -54,7 +54,7 @@
 shaka_version = shakaBuildHelpers.calculate_version()
 
 common_closure_opts = [
-    '--language_out', 'ECMASCRIPT3',
+    '--language_out', 'ECMASCRIPT5',
 
     '--jscomp_error=*',
 
diff --git a/build/conformance.textproto b/build/conformance.textproto
index d6b52453d8..a2ffb71ef2 100644
--- a/build/conformance.textproto
+++ b/build/conformance.textproto
@@ -269,6 +269,7 @@ requirement: {
   whitelist_regexp: "demo/"
   whitelist_regexp: "test/"
   whitelist_regexp: "lib/util/delayed_tick.js"
+  whitelist_regexp: "lib/polyfill/abort_controller.js"
 }
 requirement: {
   type: BANNED_NAME
diff --git a/build/types/polyfill b/build/types/polyfill
index 93e134a177..f85956aba6 100644
--- a/build/types/polyfill
+++ b/build/types/polyfill
@@ -1,6 +1,7 @@
 # Polyfills used to emulate missing browsers features.
 
 +../../node_modules/eme-encryption-scheme-polyfill/index.js
++../../lib/polyfill/abort_controller.js
 +../../lib/polyfill/aria.js
 +../../lib/polyfill/encryption_scheme.js
 +../../lib/polyfill/fullscreen.js
diff --git a/lib/polyfill/abort_controller.js b/lib/polyfill/abort_controller.js
new file mode 100644
index 0000000000..3ebf1acc6e
--- /dev/null
+++ b/lib/polyfill/abort_controller.js
@@ -0,0 +1,143 @@
+/*! @license
+ * Shaka Player
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+goog.provide('shaka.polyfill.AbortController');
+
+goog.require('shaka.polyfill');
+goog.require('shaka.util.FakeEvent');
+goog.require('shaka.util.FakeEventTarget');
+
+/**
+ * @summary A polyfill for systems that do not implement AbortController.
+ * This is used both with the fetch API for HTTP requests and inside the HLS
+ * parser.
+ * @export
+ * @extends AbortController
+ */
+shaka.polyfill.AbortController = class {
+  /**
+   * Install the polyfill if needed.
+   * @export
+   */
+  static install() {
+    if (window.AbortController) {
+      // Not needed.
+      return;
+    }
+
+    window.AbortController = shaka.polyfill.AbortController;
+    window.AbortSignal = shaka.polyfill.AbortController.AbortSignal;
+  }
+
+  /** */
+  constructor() {
+    /** @private {!shaka.polyfill.AbortController.AbortSignal} */
+    this.signal_ = new shaka.polyfill.AbortController.AbortSignal();
+  }
+
+  /**
+   * @override
+   * @suppress {const|duplicate} Since the original is defined as "const", we
+   *   need this suppression to override it.
+   */
+  get signal() {
+    return this.signal_;
+  }
+
+  /**
+   * @param {*=} reason
+   * @override
+   */
+  abort(reason) {
+    this.signal_.doAbort_(reason);
+  }
+};
+
+/**
+ * @summary A polyfill for AbortSignal, part of the AbortController API.
+ * @implements {AbortSignal}
+ */
+shaka.polyfill.AbortController.AbortSignal =
+class extends shaka.util.FakeEventTarget {
+  /** */
+  constructor() {
+    super();
+
+    /** @private {boolean} */
+    this.aborted_ = false;
+
+    /** @private {*} */
+    this.reason_ = undefined;
+
+    /** @type {?function(!Event)} */
+    this.onabort = null;
+  }
+
+  /** @override */
+  get aborted() {
+    return this.aborted_;
+  }
+
+  /** @return {*} */
+  get reason() {
+    return this.reason_;
+  }
+
+  /**
+   * @param {*} reason
+   * @private
+   */
+  doAbort_(reason) {
+    if (this.aborted_) {
+      return;
+    }
+
+    this.aborted_ = true;
+    this.reason_ = reason;
+    if (this.reason_ === undefined) {
+      // This is equivalent to a native implementation.
+      this.reason_ = new DOMException(
+          'signal is aborted without reason', 'AbortError');
+    }
+
+    // According to MDN:
+    // "Event type - A generic Event with no added properties."
+    const event = new shaka.util.FakeEvent('abort');
+    if (this.onabort) {
+      this.onabort(event);
+    }
+    this.dispatchEvent(event);
+  }
+
+
+  /**
+   * @param {*=} reason
+   * @return {!AbortSignal}
+   */
+  static abort(reason) {
+    const signal = new shaka.polyfill.AbortController.AbortSignal();
+    signal.doAbort_(reason);
+    return signal;
+  }
+
+  /**
+   * @param {number} timeMs
+   * @return {!AbortSignal}
+   */
+  static timeout(timeMs) {
+    const signal = new shaka.polyfill.AbortController.AbortSignal();
+
+    window.setTimeout(() => {
+      // This is equivalent to a native implementation.
+      signal.doAbort_(new DOMException('signal timed out', 'TimeoutError'));
+    }, timeMs);
+
+    return signal;
+  }
+};
+
+
+shaka.polyfill.register(shaka.polyfill.AbortController.install);
diff --git a/shaka-player.uncompiled.js b/shaka-player.uncompiled.js
index 91f009ceae..880f0c4d79 100644
--- a/shaka-player.uncompiled.js
+++ b/shaka-player.uncompiled.js
@@ -32,6 +32,7 @@ goog.require('shaka.offline.OfflineManifestParser');
 goog.require('shaka.offline.OfflineScheme');
 goog.require('shaka.offline.Storage');
 goog.require('shaka.offline.indexeddb.StorageMechanism');
+goog.require('shaka.polyfill.AbortController');
 goog.require('shaka.polyfill.Aria');
 goog.require('shaka.polyfill.EncryptionScheme');
 goog.require('shaka.polyfill.Fullscreen');