Script to clean release directory

You can now do npm run clean:buildDir, ... clean:releaseDir, or just
... clean, which does both.

The release directory is automatically cleaned before packaging
commences.

scripts/gulpfiles/build_tasks.js

@@ -559,8 +559,7 @@ function cleanBuildDir(done) {
   if (BUILD_DIR === '.' || BUILD_DIR === '/') {
     throw new Error(`Refusing to rm -rf ${BUILD_DIR}`);
   }
-  rimraf.sync(BUILD_DIR);
-  done();
+  rimraf(BUILD_DIR, done);
 }
 
 module.exports = {

scripts/gulpfiles/package_tasks.js

@@ -366,33 +366,47 @@ function packageDTS() {
     .pipe(gulp.dest(`${RELEASE_DIR}`));
 };
 
+/**
+ * This task cleans the release directory (by deleting it).
+ */
+function cleanReleaseDir(done) {
+  // Sanity check.
+  if (RELEASE_DIR === '.' || RELEASE_DIR === '/') {
+    throw new Error(`Refusing to rm -rf ${RELEASE_DIR}`);
+  }
+  rimraf(RELEASE_DIR, done);
+}
+
 /**
  * This task prepares the files to be included in the NPM by copying
  * them into the release directory.
  */
-const package = gulp.parallel(
-  packageIndex,
-  packageSources,
-  packageCompressed,
-  packageBrowser,
-  packageNode,
-  packageCore,
-  packageNodeCore,
-  packageBlockly,
-  packageBlocks,
-  packageJavascript,
-  packagePython,
-  packageLua,
-  packageDart,
-  packagePHP,
-  packageLocales,
-  packageMedia,
-  packageUMDBundle,
-  packageJSON,
-  packageReadme,
-  packageDTS
-);
+const package = gulp.series(
+    cleanReleaseDir,
+    gulp.parallel(
+        packageIndex,
+        packageSources,
+        packageCompressed,
+        packageBrowser,
+        packageNode,
+        packageCore,
+        packageNodeCore,
+        packageBlockly,
+        packageBlocks,
+        packageJavascript,
+        packagePython,
+        packageLua,
+        packageDart,
+        packagePHP,
+        packageLocales,
+        packageMedia,
+        packageUMDBundle,
+        packageJSON,
+        packageReadme,
+        packageDTS)
+    );
 
 module.exports = {
+  cleanReleaseDir: cleanReleaseDir,
   package: package,
 };