Make it possible to override the SourceKind on a SyntheticAst for testing.
Make it possible to override the SourceKind on a SyntheticAst for testing.

PiperOrigin-RevId: 343603819

ChadKillingsworth/closure-compiler,vobruba-martin/closure-compiler,vobruba-martin/closure-compiler,google/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,google/closure-compiler,vobruba-martin/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,monetate/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,google/closure-compiler,google/closure-compiler,nawawi/closure-compiler