Expose WakeLockManager state transitions for tests

Dispatch an event on each state transition inside the WakeLockManager.
This gives the unit tests something to synchronise on, allowing us to
write fast, flake-free tests.

Author: tsukasa-au

core/util/wakelock.js

@@ -59,8 +59,18 @@ const _STATES = {
     ERROR: 'error',
 };
 
-export default class WakeLockManager {
+class TestOnlyWakeLockManagerStateChangeEvent extends Event {
+    constructor(oldState, newState) {
+        super("testOnlyStateChange");
+        this.oldState = oldState;
+        this.newState = newState;
+    }
+}
+
+export default class WakeLockManager extends EventTarget {
     constructor() {
+        super();
+
         this._state = _STATES.RELEASED;
         this._wakelock = null;
 
@@ -130,6 +140,7 @@ export default class WakeLockManager {
         let oldState = this._state;
         Log.Debug(`WakelockManager transitioning ${oldState} -> ${newState}`);
         this._state = newState;
+        this.dispatchEvent(new TestOnlyWakeLockManagerStateChangeEvent(oldState, newState));
     }
 
     _awaitVisible() {