An issue that has arisen recently for contributors making changes to the GLSL ES grammar files has been that certain versions of flex, the lexer on which ANGLE relies, produce outputs which are not safe in 64-bit builds.
To address this issue, ANGLE has added a step to its generation scripts to apply 64-bit safety fixes to newly regenerated outputs. This should be unnoticeable to developers invoking flex via the generate_parser.sh scripts in the relevant compiler directories, as the fixes will be applied by the patch utility as part of that script.
When making code contributions that affect the grammar files, please ensure that you've generated the outputs using the script, to make certain that the 64-bit safety fixes are applied.