This is partially implemented, but still lacks majority of the rules.

Unlike graphql-js, there is no validation context given when validating a document. A schema validation rule should be self-sufficient and based only on the given AST document.
See https://github.com/gql-dart/gql/tree/master/gql/lib/src/validation.

• LoneSchemaDefinition (ref)
• UniqueOperationTypes (ref)
• UniqueTypeNames (ref; issue)
• UniqueEnumValueNames (ref)
• UniqueFieldDefinitionNames (ref)
• UniqueDirectiveNames (ref)
• KnownTypeNames (ref; issue)
• KnownDirectives (ref; issue)
• UniqueDirectivesPerLocation (ref; issue)
• PossibleTypeExtensions (ref; issue)
• KnownArgumentNamesOnDirectives (ref; issue)
• UniqueArgumentNames (ref; issue)
• UniqueInputFieldNames (ref; issue)
• ProvidedRequiredArgumentsOnDirectives (ref; issue)