Convert externs to ES6.

Issue shaka-project#1157

Change-Id: Ib8ddecc7106fea8c24c10cfac406c4709a3f275b

.eslintrc.js

@@ -205,5 +205,13 @@ module.exports = {
         'lib/debug/asserts.js',
       ],
     },
+    {
+      'files': ['externs/*', 'externs/shaka/*'],
+      'rules': {
+        // Disable rules on useless constructors so we can use ES6 classes in
+        // externs.
+        'no-useless-constructor': 'off',
+      },
+    },
   ],
 };

externs/audiotrack.js

@@ -49,18 +49,16 @@ AudioTrack.prototype.sourceBuffer;
  * @extends {EventTarget}
  * @interface
  */
-function AudioTrackList() {}
+class AudioTrackList {
+  /** @override */
+  addEventListener(type, listener, useCapture) {}
 
-/** @override */
-AudioTrackList.prototype.addEventListener =
-    function(type, listener, useCapture) {};
+  /** @override */
+  removeEventListener(type, listener, useCapture) {}
 
-/** @override */
-AudioTrackList.prototype.removeEventListener =
-    function(type, listener, useCapture) {};
-
-/** @override */
-AudioTrackList.prototype.dispatchEvent = function(event) {};
+  /** @override */
+  dispatchEvent(event) {}
+}
 
 
 /** @type {AudioTrackList} */

externs/awesomplete.js

@@ -21,16 +21,15 @@
  */
 
 
-/**
- * @param {!Element} input
- * @constructor
- */
-const Awesomplete = function(input) {};
-
-/** @type {!Array.<string>} */
-Awesomplete.prototype.list;
+class Awesomplete {
+  /** @param {!Element} input */
+  constructor(input) {
+    /** @type {!Array.<string>} */
+    this.list;
 
-/** @type {number} */
-Awesomplete.prototype.minChars;
+    /** @type {number} */
+    this.minChars;
+  }
 
-Awesomplete.prototype.evaluate = function() {};
+  evaluate() {}
+}