flutter is is not a valid Dart package name. code example

Example 1: is not a valid dart package name.

Package names should be all lowercase, with underscores to separate
words, just_like_this. Use only basic Latin letters and Arabic 
digits: [a-z0-9_]. Also, make sure the name is a valid Dart identifier 
-- that it doesn't start with digits and isn't a reserved word.

Example 2: is not a valid dart package name

This is documented on the name section of Pubspec Format documentation.

It should be all lowercase, with underscores to separate words, just_like_this. Stick with basic Latin letters and Arabic digits: [a-z0-9_] and ensure that it’s a valid Dart identifier (i.e. doesn’t start with digits and isn’t a reserved word).

Try to pick a name that is clear, terse, and not already in use.