refactor: rename private method name to symbol

[iyuq]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

rename private method use es6 symbol to make method truely private.

[mention-bot]

@iyuq, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dead-horse, @popomore and @fengmk2 to be potential reviewers.

[codecov]

Codecov Report

Merging #904 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #904   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          28     28           
  Lines         667    669    +2     
=====================================
+ Hits          667    669    +2

Impacted Files	Coverage Δ
lib/application.js	100% <100%> (ø)	⬆️

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