pub.dart compiling with stubs for io.dart.

pub.dart compiling with stubs for io.dart.

Now pub.dart is compiling (at least the files needed for 'pub get').
The tests are not though, as 'package:scheduled_test' is not able to
be compiled to JS. All directory/files currently ignored are prefixed
by '_'.

These will not compile under dart2js:
  dart:io
  dart:http
  package:analyzer (relies on dart:io)
  package:barback (relies on dart:io)
  package:http (relies on dart:io)
  package:oauth2 (relies on package:http/src/* which rely on dart:io)
  package:scheduled_test (relies on dart:io)
  package:watcher (relies on dart:io)

Extract files that will be used for 'get' command;
all non-'get' files are prefixed with '_'. References
to 'dart:io', 'dart:http', 'http.dart' are
all removed. Also Platform dependent code is commented out.