Fix travis to fail on analysis lints (dart-lang/stream_transform#91)

Fix a case of `unnecessary_this` that was missed.

pkgs/stream_transform/.travis.yml

diff --git a/pkgs/stream_transform/.travis.yml b/pkgs/stream_transform/.travis.yml
index 0bd0f9c..d69c3f7 100644
--- a/pkgs/stream_transform/.travis.yml
+++ b/pkgs/stream_transform/.travis.yml
@@ -11,4 +11,4 @@
   - test
   - test -p chrome
   - dartfmt
-  - dartanalyzer
+  - dartanalyzer: --fatal-warnings --fatal-infos .

pkgs/stream_transform/lib/src/async_map.dart

diff --git a/pkgs/stream_transform/lib/src/async_map.dart b/pkgs/stream_transform/lib/src/async_map.dart
index 464e497..f002667 100644
--- a/pkgs/stream_transform/lib/src/async_map.dart
+++ b/pkgs/stream_transform/lib/src/async_map.dart
@@ -41,8 +41,7 @@
     var workFinished = StreamController<void>()
       // Let the first event through.
       ..add(null);
-    return this
-        .buffer(workFinished.stream)
+    return buffer(workFinished.stream)
         .transform(_asyncMapThen(convert, workFinished.add));
   }