feat(build): Make appengine tasks invoke their prerequisites

In this case prepareDeployDir will eventually depend on package
but does not for now.

gulpfile.js

@@ -35,6 +35,9 @@ module.exports = {
   package: packageTasks.package,
   publish: releaseTasks.publish,
   publishBeta: releaseTasks.publishBeta,
+  prepareDemos: appengineTasks.prepareDemos,
+  deployDemos: appengineTasks.deployDemos,
+  deployDemosBeta: appengineTasks.deployDemosBeta,
   gitUpdateGithubPages: gitTasks.updateGithubPages,
 
   // Manually-invokable targets, with prequisites where required.
@@ -55,9 +58,4 @@ module.exports = {
   recompile: releaseTasks.recompile,
   gitSyncDevelop: gitTasks.syncDevelop,
   gitSyncMaster: gitTasks.syncMaster,
-
-  // TBD.
-  deployDemos: appengineTasks.deployDemos,
-  deployDemosBeta: appengineTasks.deployDemosBeta,
-  prepareDemos: appengineTasks.prepareDemos,
 };

scripts/gulpfiles/appengine_tasks.js

@@ -143,6 +143,7 @@ const deployDemos = gulp.series(prepareDemos, deployAndClean);
 const deployDemosBeta = gulp.series(prepareDemos, deployBetaAndClean);
 
 module.exports = {
+  // Main sequence targets.  Each should invoke any immediate prerequisite(s).
   deployDemos: deployDemos,
   deployDemosBeta: deployDemosBeta,
   prepareDemos: prepareDemos