Rename File.readAsText to File.readAsString. There is no Text type in Dart
and the methods return Strings.

R=sgjesse@google.com
BUG=

Review URL: https://codereview.chromium.org//11412033

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@15011 260f80e4-7a28-3924-810f-c04153c831b5