Dart CLI should check for valid project names

[mit-mit]

No reason to fail here; could check that the passed arg is a valid identifier:

mit-macbookpro4:tmp mit$ dart create gumbo-dart
Creating /Users/mit/tmp/gumbo-dart using template console-simple...

  .gitignore
  CHANGELOG.md
  README.md
  analysis_options.yaml
  bin/gumbo-dart.dart
  pubspec.yaml

Running pub get...
Error on line 1, column 7 of pubspec.yaml: "name" field must be a valid Dart identifier.
  ╷
1 │ name: gumbo-dart
  │       ^^^^^^^^^^
  ╵

[devoncarew]

For the flutter CLI validation, see https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/create.dart#L988.