This moves validation out of GraphQLSchema's constructor (but not yet from other type constructors), which is responsible for root type validation and interface implementation checking.
Reduces time to construct GraphQLSchema significantly, shifting the time to validation.
This also allows for much looser rules within the schema builders, which implicitly validate while trying to adhere to flow types. Instead we use any casts to loosen the rules to defer that to validation where errors can be richer.
This also loosens the rule that a schema can only be constructed if it has a query type, moving that to validation as well. That makes flow typing slightly less nice, but allows for incremental schema building which is valuable
ref: graphql/graphql-js#1124
