Whether the scheme of this Uri is scheme.
The scheme should be the same as the one returned by Uri.scheme,
but doesn't have to be case-normalized to lower-case characters.
Example:
var uri = Uri.parse("http://example.com/");
print(uri.isScheme("HTTP")); // Prints true.
A null or empty scheme string matches a URI with no scheme
(one where hasScheme returns false).
Source
bool isScheme(String scheme);