Allow main method to be called from elsewhere

tasks/generate-symbols.js

@@ -145,15 +145,27 @@ function writeSymbols(symbols, output, callback) {
 /**
  * Determine which source files have been changed, run JSDoc against those,
  * write out exported symbols, and clean up the build dir.
+ *
+ * @param {function(Error)} callback Called when the symbols file has been
+ *     written (or if an error occurs).
  */
+exports.main = function(callback) {
   async.waterfall([
     readSymbols,
     getNewer,
     spawnJSDoc,
     writeSymbols
-], function(err) {
+  ], callback);
+};
+
+
+if (require.main === module) {
+  exports.main(function(err) {
     if (err) {
       console.error(err.message);
       process.exit(1);
+    } else {
+      process.exit(0);
     }
   });
+}