message: Fix legit Flow error in getDiagnostics
  Discovered while trying to get Flow to work in CI.
diff: import type {GraphQLErrorLocation, GraphQLError} from 'graphql/error';
  import type {ASTNode} from 'graphql/language';
  +import type {GraphQLSchema} from 'graphql/type';
  import type {
  Diagnostic,
  CustomValidationRule,
  @@ -33,7 +34,7 @@ export const SEVERITY = {
  export function getDiagnostics(
  queryText: string,
  - schema: ?string = null,
  + schema: ?GraphQLSchema = null,
  customRules?: Array<CustomValidationRule>,
  ): Array<Diagnostic> {
  let ast = null;
  