Add Babel sourcemap to resolve VSCode debugging issues (#7022)

Author: justingrant

packages/react-scripts/config/webpack.config.js

@@ -489,11 +489,11 @@ module.exports = function(webpackEnv) {
                   ]
                 ),
                 // @remove-on-eject-end
-                // If an error happens in a package, it's possible to be
-                // because it was compiled. Thus, we don't want the browser
-                // debugger to show the original code. Instead, the code
-                // being evaluated would be much more helpful.
-                sourceMaps: false,
+                // Babel sourcemaps are needed for debugging into node_modules
+                // code.  Without the options below, debuggers like VSCode
+                // show incorrect code and set breakpoints on the wrong lines.
+                sourceMaps: shouldUseSourceMap,
+                inputSourceMap: shouldUseSourceMap,
               },
             },
             // "postcss" loader applies autoprefixer to our CSS.