Allow partially overriding webpack config

packages/@angular/cli/models/webpack-config.ts

@@ -8,7 +8,8 @@ import {
   getProdConfig,
   getStylesConfig,
   getNonAotConfig,
-  getAotConfig
+  getAotConfig,
+  getCustomConfig
 } from './webpack-configs';
 
 const path = require('path');
@@ -51,6 +52,8 @@ export class NgCliWebpackConfig {
       webpackConfigs.push(typescriptConfigPartial);
     }
 
+    webpackConfigs.push(getCustomConfig(this.wco));
+
     this.config = webpackMerge(webpackConfigs);
     return this.config;
   }

packages/@angular/cli/models/webpack-configs/custom.ts

@@ -0,0 +1,14 @@
+import * as fs from 'fs';
+import * as path from 'path';
+
+import { WebpackConfigOptions } from '../webpack-config';
+
+export const getCustomConfig = function (_wco: WebpackConfigOptions) {
+  const customConfigPath = path.join(_wco.projectRoot, 'webpack.config.js');
+
+  if (fs.existsSync(customConfigPath)) {
+    return require(customConfigPath);
+  }
+
+  return {};
+};

packages/@angular/cli/models/webpack-configs/index.ts

@@ -1,5 +1,6 @@
 export * from './browser';
 export * from './common';
+export * from './custom';
 export * from './development';
 export * from './production';
 export * from './styles';