Fix support for multi compiler in webpack 4

Ref #1301

lib/Server.js

@@ -65,12 +65,20 @@ function Server(compiler, options) {
     });
     compiler.apply(progressPlugin);
   }
-  compiler.hooks.compile.tap('webpack-dev-server', invalidPlugin);
-  compiler.hooks.invalid.tap('webpack-dev-server', invalidPlugin);
-  compiler.hooks.done.tap('webpack-dev-server', (stats) => {
-    this._sendStats(this.sockets, stats.toJson(clientStats));
-    this._stats = stats;
-  });
+
+  const addCompilerHooks = (comp) => {
+    comp.hooks.compile.tap('webpack-dev-server', invalidPlugin);
+    comp.hooks.invalid.tap('webpack-dev-server', invalidPlugin);
+    comp.hooks.done.tap('webpack-dev-server', (stats) => {
+      this._sendStats(this.sockets, stats.toJson(clientStats));
+      this._stats = stats;
+    });
+  };
+  if (compiler.compilers) {
+    compiler.compilers.forEach(addCompilerHooks);
+  } else {
+    addCompilerHooks(compiler);
+  }
 
   // Init express server
   const app = this.app = new express(); // eslint-disable-line

test/MultiCompiler.test.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const request = require('supertest');
+const helper = require('./helper');
+const config = require('./fixtures/multi-compiler-config/webpack.config');
+
+describe('MultiCompiler', () => {
+  let server;
+  let req;
+  before((done) => {
+    server = helper.start(config, {}, done);
+    req = request(server.app);
+  });
+
+  after(helper.close);
+
+  // TODO: this is a very basic test, optimally it should test multiple configs etc.
+  it('GET request to bundle', (done) => {
+    req.get('/main.js')
+      .expect('Content-Type', 'application/javascript; charset=UTF-8')
+      .expect(200, done);
+  });
+});

test/fixtures/multi-compiler-config/foo.js

@@ -0,0 +1,3 @@
+'use strict';
+
+console.log('Hey.');

test/fixtures/multi-compiler-config/webpack.config.js

@@ -0,0 +1,11 @@
+'use strict';
+
+module.exports = [{
+  mode: 'development',
+  context: __dirname,
+  entry: './foo.js',
+  output: {
+    path: '/'
+  },
+  node: false
+}];