fix(launcher): Log state transitions in debug

See karma-runner#3290

lib/launchers/base.js

@@ -5,12 +5,12 @@ const Promise = require('bluebird')
 const log = require('../logger').create('launcher')
 const helper = require('../helper')
 
-const BEING_CAPTURED = 1
-const CAPTURED = 2
-const BEING_KILLED = 3
-const FINISHED = 4
-const RESTARTING = 5
-const BEING_FORCE_KILLED = 6
+const BEING_CAPTURED = 'BEING_CAPTURED'
+const CAPTURED = 'CAPTURED'
+const BEING_KILLED = 'BEING_KILLED'
+const FINISHED = 'FINISHED'
+const RESTARTING = 'RESTARTING'
+const BEING_FORCE_KILLED = 'BEING_FORCE_KILLED'
 
 /**
  * Base launcher that any custom launcher extends.
@@ -29,7 +29,17 @@ function BaseLauncher (id, emitter) {
   this.emitAsync = KarmaEventEmitter.prototype.emitAsync.bind(this)
 
   this.id = id
-  this.state = null
+  this._state = null
+  Object.defineProperty(this, 'state', {
+    get: () => {
+      return this._state
+    },
+    set: (toState) => {
+      log.debug(`${this._state} -> ${toState}`)
+      this._state = toState
+    }
+  })
+
   this.error = null
 
   let killingPromise