Remove unneeded imports (dart-lang/stream_channel#74)

All cleanup null-safety bits from pubspec

pkgs/stream_channel/lib/src/json_document_transformer.dart

diff --git a/pkgs/stream_channel/lib/src/json_document_transformer.dart b/pkgs/stream_channel/lib/src/json_document_transformer.dart
index 237193b..3feda43 100644
--- a/pkgs/stream_channel/lib/src/json_document_transformer.dart
+++ b/pkgs/stream_channel/lib/src/json_document_transformer.dart
@@ -7,7 +7,6 @@
 import 'package:async/async.dart';
 
 import '../stream_channel.dart';
-import 'stream_channel_transformer.dart';
 
 /// A [StreamChannelTransformer] that transforms JSON documents—strings that
 /// contain individual objects encoded as JSON—into decoded Dart objects.

pkgs/stream_channel/pubspec.yaml

diff --git a/pkgs/stream_channel/pubspec.yaml b/pkgs/stream_channel/pubspec.yaml
index e7f6c7a..d4a7173 100644
--- a/pkgs/stream_channel/pubspec.yaml
+++ b/pkgs/stream_channel/pubspec.yaml
@@ -7,11 +7,11 @@
 homepage: https://github.com/dart-lang/stream_channel
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   async: ^2.5.0
 
 dev_dependencies:
-  pedantic: ^1.10.0-nullsafety
-  test: ^1.16.0-nullsafety
+  pedantic: ^1.10.0
+  test: ^1.16.0