Make Uri.parseUserPart, parseHost, and parsePort symmetric

Currently parseUserPart uses the encoded authority to split the URI
into user and non-user parts, but the parseHost and parsePort uses
the decoded URI to split the URI into non-host, host, and port parts.
This gives unexpected results when %40 ('@') and %3a (':') is used
in a URI:

Uri test = Uri.parse("http://bob%40lee%3ajr@example.com:42/");
test.getUserInfo() => "bob@lee:jr"
test.getHost() => "lee:jr@example.com" (should be "example.com")
test.getPort() => -1 (should be 42)