It now loads dart:io via mirrors if it's available. This allows it to be
compatible with the browser, so a new library "package:http/browser_client.dart"
is added to support that.
The use of mirrors was tested to ensure that it doesn't add any additional
filesize to dart2js compilations over what would be used if "dart:io" support
didn't exist.
R=rnystrom@google.com
Review URL: https://codereview.chromium.org//261763002
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@35756 260f80e4-7a28-3924-810f-c04153c831b5