Changed default Uri constructor to Uri.fromComponents(), new default constructor is Uri.fromString(…

Changed default Uri constructor to Uri.fromComponents(), new default constructor is Uri.fromString(). This is to avoid new Uri('http://www.google.com') resulting in Uri with scheme='http://www.google.com'.

Committing on behalf of: Alexander Aprelev <aprelev@gmail.com>;

BUG=dart-3348
TEST=new Uri('http://www.google.com') is the same as new Uri.fromString('http://www.google.com') same as new Uri.fromComponents(scheme:'http', domain:'www.google.com')

Committed: https://code.google.com/p/dart/source/detail?r=10168