support for sourcemaps in browser code (#5451)

changed from 'eval' to 'cheap-module-eval-source-map' to 
generate source map of the original code instead of transpiled code.

browser/package.json

@@ -4,7 +4,7 @@
   "description": "Minio Browser",
   "scripts": {
     "test": "karma start",
-    "dev": "NODE_ENV=dev webpack-dev-server --devtool eval --progress --colors --hot --content-base dev",
+    "dev": "NODE_ENV=dev webpack-dev-server --devtool cheap-module-eval-source-map --progress --colors --hot --content-base dev",
     "build": "NODE_ENV=dev node build.js",
     "release": "NODE_ENV=production MINIO_UI_BUILD=RELEASE node build.js",
     "format": "esformatter -i 'app/**/*.js'"