Remove Object.keys polyfill

[kevinoid]

Purpose (TL;DR) - mandatory

Fix inconsistent use of Object.keys and keys polyfill.

Background (Problem in detail) - optional

While considering how to implement Object.assign, I noticed that Object.keys is used in some places and the keys conditional polyfill is used in others. This intent is unclear (although after reviewing the history I assume it was left over from previous compatibility requirements) and it seemed to present needless complexity and maintenance burden.

Solution - optional

The current sinon compatibility specifies that "the source is written as ES5.1", which defines Object.keys. This PR replaces the two remaining uses of the polyfill and removes it to reduce.

Alternate Solution

If there is reason to keep the polyfill, I could send a PR to use it everywhere, move it to commons, and/or add comments to explain why the use is necessary.

[codecov[bot]]

Codecov Report

Merging #348 (bed73ac) into master (bdf8984) will increase coverage by 0.83%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
+ Coverage   93.15%   93.98%   +0.83%     
==========================================
  Files           1        1              
  Lines         555      549       -6     
==========================================
- Hits          517      516       -1     
+ Misses         38       33       -5     

Flag	Coverage Δ
unit	93.98% <100.00%> (+0.83%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/fake-timers-src.js	93.98% <100.00%> (+0.83%)	:arrow_up:

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[benjamingr]

Thank you