minor improvement to gulp generate-runtime-helpers error message (bab…

…el#13522)

* improve gulp generate-runtime-helpers error message

If you had some garbage in packages/babel-runtime-helpers/src/helpers/,
gulp generate-runtime-helpers would blow up with an unhelpful message:

    TypeError: Cannot read property 'groups' of null

* ignore files starting with a dot when generating runtime helpers

packages/babel-helpers/scripts/generate-helpers.js

@@ -17,18 +17,21 @@ import template from "@babel/template";
 
   for (const file of (await fs.promises.readdir(HELPERS_FOLDER)).sort()) {
     if (IGNORED_FILES.has(file)) continue;
+    if (file.startsWith(".")) continue; // ignore e.g. vim swap files
 
     const [helperName] = file.split(".");
     const isValidId = isValidBindingIdentifier(helperName);
     const varName = isValidId ? helperName : `_${helperName}`;
 
-    const fileContents = await fs.promises.readFile(
-      join(fileURLToPath(HELPERS_FOLDER), file),
-      "utf8"
-    );
-    const { minVersion } = fileContents.match(
+    const filePath = join(fileURLToPath(HELPERS_FOLDER), file);
+    const fileContents = await fs.promises.readFile(filePath, "utf8");
+    const minVersionMatch = fileContents.match(
       /^\s*\/\*\s*@minVersion\s+(?<minVersion>\S+)\s*\*\/\s*$/m
-    ).groups;
+    );
+    if (!minVersionMatch) {
+      throw new Error(`@minVersion number missing in ${filePath}`);
+    }
+    const { minVersion } = minVersionMatch.groups;
 
     // TODO: We can minify the helpers in production
     const source = fileContents