Fix scope hoisting (#152)

Issues: #151 facebook/create-react-app#4492

Same as 
#117
with a test case.

-----
fix #151 
closes #117

lib/plugin.js

@@ -40,10 +40,12 @@ ManifestPlugin.prototype.apply = function(compiler) {
   var outputName = path.relative(outputFolder, outputFile);
 
   var moduleAsset = function (module, file) {
-    moduleAssets[file] = path.join(
-      path.dirname(file),
-      path.basename(module.userRequest)
-    );
+    if (module.userRequest) {
+      moduleAssets[file] = path.join(
+        path.dirname(file),
+        path.basename(module.userRequest)
+      );
+    }
   };
 
   var emit = function(compilation, compileCallback) {

package.json

@@ -22,8 +22,10 @@
     "file-loader": "^1.1.11",
     "jest": "^22.4.3",
     "memory-fs": "^0.2.0",
+    "react": "^16.3.2",
     "rimraf": "^2.6.1",
     "style-loader": "^0.8.3",
+    "svgr": "^1.9.2",
     "webpack": "^3.5.2"
   },
   "files": [

spec/plugin.integration.spec.js

@@ -436,3 +436,48 @@ describe('ManifestPlugin with memory-fs', function() {
     });
   });
 });
+
+describe('scoped hoisting', function() {
+  beforeAll(function () {
+    fse.outputFileSync(path.join(__dirname, 'output/scoped-hoisting/index.js'), 'import { ReactComponent } from "./logo.svg";');
+    fse.outputFileSync(path.join(__dirname, 'output/scoped-hoisting/logo.svg'), '<svg />');
+  });
+
+  it('outputs a manifest', function(done) {
+    let plugins;
+    if (webpack.optimize.ModuleConcatenationPlugin) {
+      // ModuleConcatenationPlugin works with webpack 3, 4.
+      plugins = [
+        new webpack.optimize.ModuleConcatenationPlugin(),
+        new ManifestPlugin(),
+      ];
+    } else {
+      plugins = [
+        new ManifestPlugin(),
+      ];
+    }
+    compiler = webpackCompile({
+      context: __dirname,
+      entry: './output/scoped-hoisting/index.js',
+      module: {
+        rules: [
+          {
+            test: /\.svg$/,
+            use: ['svgr/webpack', 'file-loader']
+          },
+        ],
+      },
+      output: {
+        filename: '[name].[hash].js',
+        path: path.join(__dirname, 'output/scoped-hoisting')
+      },
+      plugins,
+    }, {}, function(stats) {
+      var manifest = JSON.parse(fse.readFileSync(path.join(__dirname, 'output/scoped-hoisting/manifest.json')))
+
+      expect(manifest).toBeDefined();
+      expect(manifest['main.js']).toEqual('main.'+ stats.hash +'.js');
+      return done();
+    });
+  });
+});