code: function getFieldDefinitionLegacy(
    schema: GraphQLSchema,
    parentType: GraphQLOutputType,
    fieldName: string,
    fieldAST: FieldNode,
  ): ?GraphQLField<mixed, mixed> {
    let schemaFieldDef = getFieldDefinitionStrict(
      schema,
      parentType,
      fieldName,
      fieldAST,
    );
    if (!schemaFieldDef) {
      const type = getRawType(parentType);
      schemaFieldDef = getFieldDefinitionLegacyImpl(
        schema,
        type,
        fieldName,
        fieldAST,
      );
    }
    return schemaFieldDef || null;
  }
docs: Find the definition of a field of the specified type, first trying
  the standard spec-compliant resolution process and falling back
  to legacy mode that supports fat interfaces.