Automated g4 rollback of changelist 188536264.
Automated g4 rollback of changelist 188536264.

*** Reason for rollback ***

Causes some "$jscomp is not defined" errors

*** Original change description ***

Update support for ES9/ES_NEXT in base.js in preparation for adding ES9 as a mode to the compiler.

RELNOTES: tweak tranpilation detection in preparation for adding ECMASCRIPT_2018 as a language mode.

***

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188549818

google/closure-library,teppeis/closure-library,teppeis/closure-library,lucidsoftware/closure-library,google/closure-library,google/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,google/closure-library,teppeis/closure-library,teppeis/closure-library,google/closure-library,lucidsoftware/closure-library