fix(build): Skip npm prepare when running in CI (google#6244)

Have npm prepare do nothing when running in CI.

We don't need to do any building, because npm test will build
everything needed in the workflows in which it is run, and we
don't want to build anything in other workflows because a tsc
error would prevent those workflows from completing.

gulpfile.js

@@ -31,7 +31,7 @@ module.exports = {
   buildCompiled: buildTasks.compiled,
   buildAdvancedCompilationTest: buildTasks.advancedCompilationTest,
   buildJavaScript: buildTasks.javaScript,
-  buildJavaScriptAndDeps: gulp.series(buildTasks.javaScript, buildTasks.deps),
+  buildJavaScriptAndDeps: buildTasks.javaScriptAndDeps,
   // TODO(5621): Re-enable once typings generation is fixed.
   // checkin: gulp.parallel(buildTasks.checkinBuilt, typings.checkinTypings),
   checkin: gulp.parallel(buildTasks.checkinBuilt),
@@ -48,6 +48,7 @@ module.exports = {
   // typings: gulp.series(typings.typings, typings.msgTypings),
   // checkinTypings: typings.checkinTypings,
   package: packageTasks.package,
+  prepare: buildTasks.prepare,
   checkLicenses: licenseTasks.checkLicenses,
   recompile: releaseTasks.recompile,
   prepareDemos: appengineTasks.prepareDemos,

package.json

@@ -42,7 +42,7 @@
     "license": "gulp checkLicenses",
     "lint": "eslint .",
     "package": "gulp package",
-    "prepare": "gulp buildJavaScriptAndDeps",
+    "prepare": "gulp prepare",
     "prepareDemos": "gulp prepareDemos",
     "publish": "gulp publish",
     "publish:beta": "gulp publishBeta",

scripts/gulpfiles/build_tasks.js

@@ -290,6 +290,26 @@ var JSCOMP_OFF = [
   'visibility',
 ];
 
+/**
+ * The npm prepare script, run by npm install after installing modules
+ * and as part of the packaging process.
+ *
+ * This does just enough of the build that npm start should work.
+ *
+ * Exception: when running in the CI environment, we don't build
+ * anything.  We don't need to, because npm test will build everything
+ * needed, and we don't want to, because a tsc error would prevent
+ * other workflows (like lint and format) from completing.
+ */
+function prepare() {
+  if (process.env.CI) {
+    return gulp.src('.');  // Do nothing.
+  }
+  return buildJavaScriptAndDeps();
+}
+
+const buildJavaScriptAndDeps = gulp.series(buildJavaScript, buildDeps);
+
 /**
  * Builds Blockly as a JS program, by running tsc on all the files in
  * the core directory.  This must be run before buildDeps or
@@ -728,7 +748,9 @@ function format() {
 }
 
 module.exports = {
+  prepare: prepare,
   build: build,
+  javaScriptAndDeps: buildJavaScriptAndDeps,
   javaScript: buildJavaScript,
   deps: buildDeps,
   generateLangfiles: generateLangfiles,