Add missing 'const' to variable binding for 'e'.
Add missing 'const' to variable binding for 'e'.

This is preparation for switching base_test.js to a strict goog.module.

RELNOTES: n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=279198080

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