Rip out dart:io from pkg/http wherever possible.

pkg/http/lib/src/exception.dart

Index: pkg/http/lib/src/exception.dart
diff --git a/pkg/http/lib/src/exception.dart b/pkg/http/lib/src/exception.dart
new file mode 100644
index 0000000000000000000000000000000000000000..bb199d8c1741744b187e05753dbb92c3ba22597b
--- /dev/null
+++ b/pkg/http/lib/src/exception.dart
@@ -0,0 +1,17 @@
+// 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.exception;
+
+/// An exception caused by an error in a pkg/http client.
+class ClientException implements Exception {
+  final String message;
+
+  /// The URL of the HTTP request or response that failed.
+  final Uri uri;
+
+  ClientException(this.message, [this.uri]);
+
+  String toString() => message;
+}