Drop support for old SDK and shelf versions.

lib/src/util/isolate_wrapper.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// 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.
-
-library test.util.isolate_wrapper;
-
-import 'dart:async';
-import 'dart:isolate';
-
-// TODO(nweiz): Get rid of this when issue 6610 is fixed.
-/// This is a wrapper around an [Isolate] that supports a callback that will
-/// fire when [Isolate.exit] is called.
-///
-/// This is necessary to delete the source directory of the isolate only once
-/// the Isolate completes. Note that the callback won't necessarily fire before
-/// the Isolate is killed, but it comes close enough for our purposes.
-///
-/// This avoids implementing Isolate because there's no interface that's
-/// compatible with both Dart before 1.11 and Dart after 1.11.
-class IsolateWrapper {
-  final Isolate _inner;
-
-  final Function _onExit;
-
-  Capability get pauseCapability => _inner.pauseCapability;
-  SendPort get controlPort => _inner.controlPort;
-  Stream get errors => _inner.errors;
-  Capability get terminateCapability => _inner.terminateCapability;
-
-  IsolateWrapper(this._inner, this._onExit);
-
-  void addErrorListener(SendPort port) => _inner.addErrorListener(port);
-  void addOnExitListener(SendPort port) => _inner.addOnExitListener(port);
-  Capability pause([Capability resumeCapability]) =>
-      _inner.pause(resumeCapability);
-  void ping(SendPort responsePort) => _inner.ping(responsePort);
-  void removeErrorListener(SendPort port) => _inner.removeErrorListener(port);
-  void removeOnExitListener(SendPort port) => _inner.removeOnExitListener(port);
-  void resume(Capability resumeCapability) => _inner.resume(resumeCapability);
-  void setErrorsFatal(bool errorsAreFatal) =>
-      _inner.setErrorsFatal(errorsAreFatal);
-  String toString() => _inner.toString();
-
-  void kill() {
-    _inner.kill();
-    _onExit();
-  }
-}