Remove unused dart:async imports.

As of Dart 2.1, Future/Stream have been exported from dart:core.
diff --git a/pkgs/test_core/lib/src/runner/compiler_pool.dart b/pkgs/test_core/lib/src/runner/compiler_pool.dart
index 9bc4f7b..50cf577 100644
--- a/pkgs/test_core/lib/src/runner/compiler_pool.dart
+++ b/pkgs/test_core/lib/src/runner/compiler_pool.dart
@@ -4,7 +4,6 @@
 //
 // @dart=2.9
 
-import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:isolate';
diff --git a/pkgs/test_core/lib/src/runner/spawn_hybrid.dart b/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
index e9cacd6..e5043af 100644
--- a/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
+++ b/pkgs/test_core/lib/src/runner/spawn_hybrid.dart
@@ -4,7 +4,6 @@
 //
 // @dart=2.9
 
-import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';