ci: verify JS build artifacts aren't committed

Building our JS packages dirties the repo. This makes sure we don't
accidentally commit these to the repo, as it'll break Flow tests with an
obscure error message.

Changelog: [Internal]

Testing:

Ran this locally, the GH lint action should run on this PR.
I'll intentionally add a build artifact to validate.

scripts/build/build.js

@@ -36,13 +36,14 @@ const config = {
   allowPositionals: true,
   options: {
     help: {type: 'boolean'},
+    check: {type: 'boolean'},
   },
 };
 
 async function build() {
   const {
     positionals: packageNames,
-    values: {help},
+    values: {help, check},
   } = parseArgs(config);
 
   if (help) {
@@ -58,17 +59,35 @@ async function build() {
     return;
   }
 
-  console.log('\n' + chalk.bold.inverse('Building packages') + '\n');
+  if (!check) {
+    console.log('\n' + chalk.bold.inverse('Building packages') + '\n');
+  }
 
   const packagesToBuild = packageNames.length
     ? packageNames.filter(packageName => packageName in buildConfig.packages)
     : Object.keys(buildConfig.packages);
 
+  let ok = true;
   for (const packageName of packagesToBuild) {
-    await buildPackage(packageName);
+    if (check) {
+      ok &&= await checkPackage(packageName);
+    } else {
+      await buildPackage(packageName);
+    }
   }
 
-  process.exitCode = 0;
+  process.exitCode = ok ? 0 : 1;
+}
+
+async function checkPackage(packageName /*: string */) /*: Promise<boolean> */ {
+  const artifacts = await exportedBuildArtifacts(packageName);
+  if (artifacts.length > 0) {
+    console.log(
+      `${chalk.bgRed(packageName)}: has been build and the ${chalk.bold('build artifacts')} committed to the repository. This will break Flow checks.`,
+    );
+    return false;
+  }
+  return true;
 }
 
 async function buildPackage(packageName /*: string */) {
@@ -183,6 +202,33 @@ type PackageJson = {
 };
 */
 
+function isStringOnly(entries /*: mixed */) /*: entries is string */ {
+  return typeof entries === 'string';
+}
+
+async function exportedBuildArtifacts(
+  packageName /*: string */,
+) /*: Promise<string[]> */ {
+  const packagePath = path.resolve(PACKAGES_DIR, packageName, 'package.json');
+  const pkg /*: PackageJson */ = JSON.parse(
+    await fs.readFile(packagePath, 'utf8'),
+  );
+  if (pkg.exports == null) {
+    throw new Error(
+      packageName +
+        ' does not define an "exports" field in its package.json. As part ' +
+        'of the build setup, this field must be used in order to rewrite ' +
+        'paths to built files in production.',
+    );
+  }
+
+  return Object.values(pkg.exports)
+    .filter(isStringOnly)
+    .filter(filepath =>
+      path.dirname(filepath).split(path.sep).includes(BUILD_DIR),
+    );
+}
+
 /**
  * Get the set of Flow entry points to build.
  *

scripts/run-ci-javascript-tests.js

@@ -47,6 +47,13 @@ try {
     throw Error(exitCode);
   }
 
+  describe('Test: No JS build artifacts');
+  if (exec(`${YARN_BINARY} run build --check`).code) {
+    echo('Failed, there are build artifacts in this commit.');
+    exitCode = 1;
+    throw Error(exitCode);
+  }
+
   describe('Test: Flow check');
   if (exec(`${YARN_BINARY} run flow-check`).code) {
     echo('Failed to run flow.');