fix(benchmarks): use explicit main() function and do not reexport

Otherwise `pub` would not pick it up e.g. for `dart2js`.

Author: tbosch

modules/benchmarks/src/change_detection/main.js

@@ -1 +1,5 @@
-export {main} from './change_detection_benchmark';
+import * as change_detection_benchmark from './change_detection_benchmark';
+
+export function main() {
+  change_detection_benchmark.main();
+}