Add externs for `chai.util`.
Add externs for `chai.util`.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237355888

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