Remove unused dart:async imports.

As of Dart 2.1, Future/Stream have been exported from dart:core.

More information: go/dart-lsc-remove-unused-async-imports

PiperOrigin-RevId: 343863452
diff --git a/lib/support/async.dart b/lib/support/async.dart
index dae3497..2f71642 100644
--- a/lib/support/async.dart
+++ b/lib/support/async.dart
@@ -14,7 +14,7 @@
 
 library webdriver.support.async;
 
-import 'dart:async' show Completer, Future, FutureOr;
+import 'dart:async' show Completer, FutureOr;
 
 import 'package:matcher/matcher.dart' as m;
 import 'package:stack_trace/stack_trace.dart' show Chain;
diff --git a/lib/support/stdio_stepper.dart b/lib/support/stdio_stepper.dart
index b7e4bc4..b2cb904 100644
--- a/lib/support/stdio_stepper.dart
+++ b/lib/support/stdio_stepper.dart
@@ -14,7 +14,7 @@
 
 library webdriver.support.stdio_stepper;
 
-import 'dart:async' show Future, Stream, StreamController;
+import 'dart:async' show StreamController;
 import 'dart:convert' show Encoding, json;
 import 'dart:io' show exit, Stdin, stdin, systemEncoding;