The streams API implementation is behind a compilation flag. It is turned on by default and can be turned off using the --no-streams-api option in Tools/Scripts/build-webkit. This works for cmake build system but not for XCode build systems.
As I wrote on comment 56 of bug 138967, we need to put a 0 on FeatureDefines.h so that build-webkit parameter takes effect, otherwise it will be always activated. Any XCode expert in the room?
This is no longer relevant.