refactor: use Symbol and boolean

Author: ylemkimon

lib/Server.js

@@ -42,10 +42,11 @@ class Server {
   constructor(compiler, options = {}) {
     validate(schema, options, 'webpack Dev Server');
 
-    if (compiler.devServer) {
+    const devServerSymbol = Symbol.for('webpack-dev-server');
+    if (compiler[devServerSymbol]) {
       throw new Error('The compiler already has a dev server attached.');
     }
-    compiler.devServer = this;
+    compiler[devServerSymbol] = true;
 
     this.compiler = compiler;
     this.options = options;

test/server/Server.test.js

@@ -86,11 +86,11 @@ describe('Server', () => {
   });
 
   describe('devServer property of the compiler', () => {
-    it('should reference the Server instance', (done) => {
+    it('should be true', (done) => {
       const compiler = webpack(config);
       const server = new Server(compiler, baseDevConfig);
 
-      expect(compiler.devServer).toBe(server);
+      expect(compiler[Symbol.for('webpack-dev-server')]).toBe(true);
 
       compiler.hooks.done.tap('webpack-dev-server', () => {
         server.close(done);