Revert "Revert import removals (#1420)" (#1432)

This reverts commit 01bfee94c654816b8fa53f07f28bfe84b81e7f6e.
diff --git a/pkgs/test_api/CHANGELOG.md b/pkgs/test_api/CHANGELOG.md
index 60d27f4..eede7a6 100644
--- a/pkgs/test_api/CHANGELOG.md
+++ b/pkgs/test_api/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 0.2.20-dev
+
 ## 0.2.19
 
 * Stable release for null safety.
diff --git a/pkgs/test_api/lib/src/backend/metadata.dart b/pkgs/test_api/lib/src/backend/metadata.dart
index f63bcf7..0e64f8d 100644
--- a/pkgs/test_api/lib/src/backend/metadata.dart
+++ b/pkgs/test_api/lib/src/backend/metadata.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:collection';
-
 import 'package:boolean_selector/boolean_selector.dart';
 import 'package:collection/collection.dart';
 
diff --git a/pkgs/test_api/lib/src/frontend/async_matcher.dart b/pkgs/test_api/lib/src/frontend/async_matcher.dart
index 880901d..56ad776 100644
--- a/pkgs/test_api/lib/src/frontend/async_matcher.dart
+++ b/pkgs/test_api/lib/src/frontend/async_matcher.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:matcher/matcher.dart';
 
 import '../backend/invoker.dart';
diff --git a/pkgs/test_api/lib/src/frontend/expect.dart b/pkgs/test_api/lib/src/frontend/expect.dart
index cf3d927..3b5b4c6 100644
--- a/pkgs/test_api/lib/src/frontend/expect.dart
+++ b/pkgs/test_api/lib/src/frontend/expect.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:matcher/matcher.dart';
 
 import '../backend/closed_exception.dart';
diff --git a/pkgs/test_api/lib/src/frontend/future_matchers.dart b/pkgs/test_api/lib/src/frontend/future_matchers.dart
index efd9e76..696a9f1 100644
--- a/pkgs/test_api/lib/src/frontend/future_matchers.dart
+++ b/pkgs/test_api/lib/src/frontend/future_matchers.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:matcher/matcher.dart';
 
 import '../utils.dart';
diff --git a/pkgs/test_api/lib/src/frontend/stream_matcher.dart b/pkgs/test_api/lib/src/frontend/stream_matcher.dart
index 57dcf56..f9305a6 100644
--- a/pkgs/test_api/lib/src/frontend/stream_matcher.dart
+++ b/pkgs/test_api/lib/src/frontend/stream_matcher.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:async/async.dart';
 import 'package:matcher/matcher.dart';
 
diff --git a/pkgs/test_api/lib/src/frontend/stream_matchers.dart b/pkgs/test_api/lib/src/frontend/stream_matchers.dart
index e60bc58..869e6b4 100644
--- a/pkgs/test_api/lib/src/frontend/stream_matchers.dart
+++ b/pkgs/test_api/lib/src/frontend/stream_matchers.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:async/async.dart';
 import 'package:matcher/matcher.dart';
 
diff --git a/pkgs/test_api/lib/src/frontend/utils.dart b/pkgs/test_api/lib/src/frontend/utils.dart
index a2314c7..ac023e5 100644
--- a/pkgs/test_api/lib/src/frontend/utils.dart
+++ b/pkgs/test_api/lib/src/frontend/utils.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 /// Returns a [Future] that completes after the [event loop][] has run the given
 /// number of [times] (20 by default).
 ///
diff --git a/pkgs/test_api/pubspec.yaml b/pkgs/test_api/pubspec.yaml
index 6357425..ac517c6 100644
--- a/pkgs/test_api/pubspec.yaml
+++ b/pkgs/test_api/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test_api
-version: 0.2.19
+version: 0.2.20-dev
 description: A library for writing Dart tests.
 homepage: https://github.com/dart-lang/test/blob/master/pkgs/test_api
 
diff --git a/pkgs/test_api/test/frontend/expect_test.dart b/pkgs/test_api/test/frontend/expect_test.dart
index 59842f2..ee8d243 100644
--- a/pkgs/test_api/test/frontend/expect_test.dart
+++ b/pkgs/test_api/test/frontend/expect_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:test/test.dart';
 
 import '../utils.dart';
diff --git a/pkgs/test_api/test/frontend/never_called_test.dart b/pkgs/test_api/test/frontend/never_called_test.dart
index c82cf8f..e9170f9 100644
--- a/pkgs/test_api/test/frontend/never_called_test.dart
+++ b/pkgs/test_api/test/frontend/never_called_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:term_glyph/term_glyph.dart' as glyph;
 import 'package:test/test.dart';
 
diff --git a/pkgs/test_api/test/frontend/tear_down_all_test.dart b/pkgs/test_api/test/frontend/tear_down_all_test.dart
index 83ca586..d6d209f 100644
--- a/pkgs/test_api/test/frontend/tear_down_all_test.dart
+++ b/pkgs/test_api/test/frontend/tear_down_all_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
-
 import 'package:async/async.dart';
 import 'package:test/test.dart';
 
diff --git a/pkgs/test_api/test/utils.dart b/pkgs/test_api/test/utils.dart
index 73e8c84..27082af 100644
--- a/pkgs/test_api/test/utils.dart
+++ b/pkgs/test_api/test/utils.dart
@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:async';
 import 'dart:collection';
 
 import 'package:test_api/src/backend/declarer.dart';