Script to check licenses (#3757)

* Add script task to check licenses of all dependenies

scripts/gulpfiles/license_tasks.js

@@ -0,0 +1,21 @@
+/**
+ * @license
+ * Copyright 2018 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Gulp tasks to check the licenses of Blockly depenedencies.
+ */
+
+const jsgl = require('js-green-licenses');
+
+function checkLicenses() {
+  const checker = new jsgl.LicenseChecker();
+  checker.setDefaultHandlers();
+  return checker.checkLocalDirectory('.');
+};
+
+module.exports = {
+  checkLicenses: checkLicenses
+};