refactor: consolidate all events in Events.js (#3772)

This will allow us to break all dependency cycles that were forcing
us to put many things in a single file (e.g. ExecutionContext and
ElementHandle).

lib/NetworkManager.js

@@ -15,6 +15,7 @@
  */
 const EventEmitter = require('events');
 const {helper, assert, debugError} = require('./helper');
+const {Events} = require('./Events');
 const Multimap = require('./Multimap');
 
 class NetworkManager extends EventEmitter {
@@ -206,7 +207,7 @@ class NetworkManager extends EventEmitter {
     const frame = event.frameId && this._frameManager ? this._frameManager.frame(event.frameId) : null;
     const request = new Request(this._client, frame, interceptionId, this._userRequestInterceptionEnabled, event, redirectChain);
     this._requestIdToRequest.set(event.requestId, request);
-    this.emit(NetworkManager.Events.Request, request);
+    this.emit(Events.NetworkManager.Request, request);
   }
 
 
@@ -230,8 +231,8 @@ class NetworkManager extends EventEmitter {
     response._bodyLoadedPromiseFulfill.call(null, new Error('Response body is unavailable for redirect responses'));
     this._requestIdToRequest.delete(request._requestId);
     this._attemptedAuthentications.delete(request._interceptionId);
-    this.emit(NetworkManager.Events.Response, response);
-    this.emit(NetworkManager.Events.RequestFinished, request);
+    this.emit(Events.NetworkManager.Response, response);
+    this.emit(Events.NetworkManager.RequestFinished, request);
   }
 
   /**
@@ -244,7 +245,7 @@ class NetworkManager extends EventEmitter {
       return;
     const response = new Response(this._client, request, event.response);
     request._response = response;
-    this.emit(NetworkManager.Events.Response, response);
+    this.emit(Events.NetworkManager.Response, response);
   }
 
   /**
@@ -263,7 +264,7 @@ class NetworkManager extends EventEmitter {
       request.response()._bodyLoadedPromiseFulfill.call(null);
     this._requestIdToRequest.delete(request._requestId);
     this._attemptedAuthentications.delete(request._interceptionId);
-    this.emit(NetworkManager.Events.RequestFinished, request);
+    this.emit(Events.NetworkManager.RequestFinished, request);
   }
 
   /**
@@ -281,7 +282,7 @@ class NetworkManager extends EventEmitter {
       response._bodyLoadedPromiseFulfill.call(null);
     this._requestIdToRequest.delete(request._requestId);
     this._attemptedAuthentications.delete(request._interceptionId);
-    this.emit(NetworkManager.Events.RequestFailed, request);
+    this.emit(Events.NetworkManager.RequestFailed, request);
   }
 }
 
@@ -732,13 +733,6 @@ class SecurityDetails {
   }
 }
 
-NetworkManager.Events = {
-  Request: 'request',
-  Response: 'response',
-  RequestFailed: 'requestfailed',
-  RequestFinished: 'requestfinished',
-};
-
 const statusTexts = {
   '100': 'Continue',
   '101': 'Switching Protocols',