Use fs.mkdirSync instead of execSync('mkdir -p ...')

Also consistently use {recursive: true} in case directory is nested
in an also-not-yet-existent directory.

scripts/gulpfiles/appengine_tasks.js

@@ -28,8 +28,8 @@ function prepareDeployDir(done) {
   if (fs.existsSync(demoTmpDir)) {
     rimraf.sync(demoTmpDir);
   }
-  fs.mkdirSync(demoStaticTmpDir, { recursive: true });
-  done()
+  fs.mkdir(demoStaticTmpDir, {recursive: true});
+  done();
 }
 
 /**

scripts/gulpfiles/build_tasks.js

@@ -448,7 +448,7 @@ function buildLangfiles(done) {
   // Create output directory.
   // TODO(#5000): does mkidr -p work on Windows?
   const outputDir = path.join(BUILD_DIR, 'msg', 'js');
-  execSync(`mkdir -p ${outputDir}`, {stdio: 'inherit'});
+  fs.mkdirSync(outputDir, {recursive: true});
   
   // Run create_messages.py.
   let json_files = fs.readdirSync(path.join('msg', 'json'));

scripts/gulpfiles/package_tasks.js

@@ -374,7 +374,7 @@ function packageJSON(cb) {
   const json = Object.assign({}, packageJson);
   delete json['scripts'];
   if (!fs.existsSync(RELEASE_DIR)) {
-    fs.mkdirSync(RELEASE_DIR);
+    fs.mkdirSync(RELEASE_DIR, {recursive: true});
   }
   fs.writeFileSync(`${RELEASE_DIR}/package.json`,
       JSON.stringify(json, null, 2));