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

[Bob Nystrom, 2014/05/01 18:30:12]

Doc comments on the public stuff in here.

[nweiz, 2014/05/01 19:34:54]

Done.

+// 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';
+
+final _library = _getLibrary();
+
+bool get supported => _library != null;

[Bob Nystrom, 2014/05/01 18:30:12]

Nit, but how about putting all of the private fields together and having this
above or below them all?

[nweiz, 2014/05/01 19:34:54]

Done.

+
+final ClassMirror _httpClient =
+    _library.declarations[const Symbol('HttpClient')];
+
+final ClassMirror _httpException =
+    _library.declarations[const Symbol('HttpException')];
+
+final ClassMirror _file = _library.declarations[const Symbol('File')];
+
+void assertSupported(String name) {
+  if (supported) return;
+  throw new UnsupportedError("$name isn't supported on this platform.");
+}
+
+newHttpClient() => _httpClient.newInstance(const Symbol(''), []).reflectee;
+
+newFile(String path) => _file.newInstance(const Symbol(''), [path]).reflectee;
+
+bool isHttpException(error) => reflect(error).type.isSubtypeOf(_httpException);
+
+LibraryMirror _getLibrary() {
+  try {
+    return currentMirrorSystem().findLibrary(const Symbol('dart.io'));
+  } catch (_) {
+    // TODO(nweiz): narrow the catch clause when issue 18532 is fixed.
+    return null;
+  }
+}