Bump core-js from 2.5.5 to 2.6.4 in /frontend

[dependabot-preview]

Bumps core-js from 2.5.5 to 2.6.4.

Release notes

Sourced from core-js's releases.

2.6.4 - 2019.02.07

• Added a workaround against crushing an old IE11.0.9600.16384 build, #485.

2.6.3 - 2019.01.22

• Added a workaround for babel-minify bug, #479

2.6.2 - 2019.01.10

• Fixed handling of $ in String#replace, #471

2.6.1 - 2018.12.18

• Fixed an issue with minified version, #463, #465

2.6.0 - 2018.12.05

• Add direct .exec calling to RegExp#{@[**replace**](https://github.com/replace), @[**split**](https://github.com/split), @[**match**](https://github.com/match), @[**search**](https://github.com/search)}. Also, added fixes for RegExp#exec method. #411, #428, #434, #435, #453, #458, thanks [nicolo-ribaudo](https://github.com/nicolo-ribaudo).

2.5.7 - 2018.05.26

• Get rid of reserved variable name final, related #400

2.5.6 - 2018.05.07

• Forced replace native Promise in V8 6.6 (Node 10 and Chrome 66) because of a bug with resolving custom thenables
• Added a workaround for usage buggy native LG WebOS 2 Promise in microtask implementation, #396
• Added modern version internal debugging information about used versions

Changelog

Sourced from core-js's changelog.

2.6.4 - 2019.02.07

• Added a workaround against crushing an old IE11.0.9600.16384 build, #485.

2.6.3 - 2019.01.22

• Added a workaround for babel-minify bug, #479

2.6.2 - 2019.01.10

• Fixed handling of $ in String#replace, #471

2.6.1 - 2018.12.18

• Fixed an issue with minified version, #463, #465

2.6.0 - 2018.12.05

• Add direct .exec calling to RegExp#{@[**replace**](https://github.com/replace), @[**split**](https://github.com/split), @[**match**](https://github.com/match), @[**search**](https://github.com/search)}. Also, added fixes for RegExp#exec method. #428, #435, #458, thanks [nicolo-ribaudo](https://github.com/nicolo-ribaudo).

2.5.7 - 2018.05.26

• Get rid of reserved variable name final, related #400

2.5.6 - 2018.05.07

• Forced replace native Promise in V8 6.6 (Node 10 and Chrome 66) because of a bug with resolving custom thenables
• Added a workaround for usage buggy native LG WebOS 2 Promise in microtask implementation, #396
• Added modern version internal debugging information about used versions

Commits

• 8d954ed 2.6.4
• 0fc0d2a rebuild
• 75ca2ae add a workaround for IE11 crash, fix #485
• 3fb5e27 fix a typo
• ea6a01b 2.6.3
• 8a9ff17 add a workaround for babel-minify bug, fix #479
• 99ea4e2 2.6.2
• 665f4bb bump the year
• 93e0ffb Merge pull request #474 from nicolo-ribaudo/v2-fix-471
• bc3feb1 Backport #472 (Fix #471)
• Additional commits viewable in compare view

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