Extract some utilities into a separate package (#723)

* Extract some utilities into a separate package

* Add utils dir to `files` in package.json

* Do not create an empty `utils` dir on eject

packages/react-dev-utils/WatchMissingNodeModulesPlugin.js

@@ -0,0 +1,37 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+// This Webpack plugin ensures `npm install <library>` forces a project rebuild.
+// We’re not sure why this isn't Webpack's default behavior.
+// See https://github.com/facebookincubator/create-react-app/issues/186.
+
+'use strict';
+
+class WatchMissingNodeModulesPlugin {
+  constructor(nodeModulesPath) {
+    this.nodeModulesPath = nodeModulesPath;
+  }
+
+  apply(compiler) {
+    compiler.plugin('emit', (compilation, callback) => {
+      var missingDeps = compilation.missingDependencies;
+      var nodeModulesPath = this.nodeModulesPath;
+
+      // If any missing files are expected to appear in node_modules...
+      if (missingDeps.some(file => file.indexOf(nodeModulesPath) !== -1)) {
+        // ...tell webpack to watch node_modules recursively until they appear.
+        compilation.contextDependencies.push(nodeModulesPath);
+      }
+
+      callback();
+    });
+  }
+}
+
+module.exports = WatchMissingNodeModulesPlugin;