fix(webpack): Fix Webpack compilation on Windows (kriasoft#1578)

Author: jorrit

tools/webpack.config.js

@@ -17,7 +17,7 @@ import overrideRules from './lib/overrideRules';
 import pkg from '../package.json';
 
 const ROOT_DIR = path.resolve(__dirname, '..');
-const SRC_DIR = `${ROOT_DIR}/src`;
+const SRC_DIR = path.join(ROOT_DIR, 'src');
 const BUILD_DIR = `${ROOT_DIR}/build`;
 const NODE_MODULES_DIR = `${ROOT_DIR}/node_modules`;