Rip out the last dart:io dependency from pkg/http.

pkg/http/lib/src/io.dart

+// Copyright (c) 2014, 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 http.io;
+
+@MirrorsUsed(targets: const ['dart.io.HttpClient', 'dart.io.HttpException',
+  'dart.io.File'])
+import 'dart:mirrors';
+
+/// Whether `dart:io` is supported on this platform.
+bool get supported => _library != null;
+
+/// The `dart:io` library mirror, or `null` if it couldn't be loaded.
+final _library = _getLibrary();
+
+/// The `dart:io` HttpClient class mirror.
+final ClassMirror _httpClient =
+    _library.declarations[const Symbol('HttpClient')];
+
+/// The `dart:io` HttpException class mirror.
+final ClassMirror _httpException =
+    _library.declarations[const Symbol('HttpException')];
+
+/// The `dart:io` File class mirror.
+final ClassMirror _file = _library.declarations[const Symbol('File')];
+
+/// Asserts that the [name]d `dart:io` feature is supported on this platform.
+///
+/// If `dart:io` doesn't work on this platform, this throws an
+/// [UnsupportedError].
+void assertSupported(String name) {
+  if (supported) return;
+  throw new UnsupportedError("$name isn't supported on this platform.");
+}
+
+/// Creates a new `dart:io` HttpClient instance.
+newHttpClient() => _httpClient.newInstance(const Symbol(''), []).reflectee;
+
+/// Creates a new `dart:io` File instance with the given [path].
+newFile(String path) => _file.newInstance(const Symbol(''), [path]).reflectee;
+
+/// Returns whether [error] is a `dart:io` HttpException.
+bool isHttpException(error) => reflect(error).type.isSubtypeOf(_httpException);
+
+/// Tries to load `dart:io` and returns `null` if it fails.
+LibraryMirror _getLibrary() {
+  try {
+    return currentMirrorSystem().findLibrary(const Symbol('dart.io'));
+  } catch (_) {
+    // TODO(nweiz): narrow the catch clause when issue 18532 is fixed.
+    return null;
+  }
+}