Update appengine_tasks to include playground dependencies (#4591)

* Update appengine_tasks to include playground dependencies

scripts/gulpfiles/appengine_tasks.js

@@ -53,6 +53,18 @@ function copyAppengineSrc() {
   return gulp.src(appengineSrc).pipe(gulp.dest(demoTmpDir));
 }
 
+/**
+ * Copies playground deps into deploy directory.
+ */
+function copyPlaygroundDeps() {
+  const playgroundDeps = [
+      './node_modules/@blockly/dev-tools/dist/index.js',
+      './node_modules/@blockly/theme-modern/dist/index.js',
+      './node_modules/@blockly/block-test/dist/index.js',
+  ];
+  return gulp.src(playgroundDeps, {base: '.'}).pipe(gulp.dest(demoStaticTmpDir));
+}
+
 /**
  * Deploys files from tmp directory to appengine to version based on the version
  * specified in package.json and then cleans the tmp directory.
@@ -82,6 +94,7 @@ const deployDemos = gulp.series(
     prepareDeployDir,
     copyStaticSrc,
     copyAppengineSrc,
+    copyPlaygroundDeps,
     deployAndClean
 );