$ ./bin/uglifyjs -c --define FOO=false
FOO=true
^D
!1=!0;

In assetgraph-builder I have a similar situation where I use this helper function to avoid doing replacements in left-hand sides of assignments: https://github.com/One-com/assetgraph-builder/blob/master/lib/transforms/cloneForEachLocale.js#L9-L20