Update usages of implements StreamTransformer to extends StreamTransformerBase as part of the Dart 2.0 core lib changes.

More details at https://gist.github.com/lrhn/b99fe5f73a10b3b1a91579853056d08b#streamtransformers-t

PiperOrigin-RevId: 185409295

Author: keertip

lib/src/utils/async/rate_limit.dart

@@ -43,7 +43,7 @@ class _RateLimitSink implements EventSink {
 
 /// A [StreamTransformer] to rate-limit streams with a given
 /// [RateLimitStrategy] and [Duration].
-class RateLimitTransformer<S, T> implements StreamTransformer<S, T> {
+class RateLimitTransformer<S, T> extends StreamTransformerBase<S, T> {
   final Duration _duration;
   final RateLimitStrategy _rateLimitStrategy;