these synthetic classes are found by `FirBuiltinSyntheticFunctionInterfaceProvider`, which isn't based on stubs and has no explicit source This fixes tests in IJ repo, such as uast based tests: `FirLegacyUastResolveEverythingTestGenerated#testLambdaReturn` Merge-request: KT-MR-10406 Merged-by: Anna Kozlova <Anna.Kozlova@jetbrains.com>