Update web-component-tester to version 6.0.0 🚀

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Hello lovely humans,

web-component-tester just published its new version 6.0.0.

State	Update 🚀
Dependency	web-component-tester
New version	6.0.0
Type	devDependency

This version is not covered by your current version range.

Without accepting this pull request your project will work just like it did before. There might be a bunch of new features, fixes and perf improvements that the maintainers worked on for you though.

I recommend you look into these changes and try to get onto the latest version of web-component-tester.
Given that you have a decent test suite, a passing build is a strong indicator that you can take advantage of these changes by merging the proposed change into your project. Otherwise this branch is a great starting point for you to work on the update.

Do you have any ideas how I could improve these pull requests? Did I report anything you think isn’t right?
Are you unsure about how things are supposed to work?

There is a collection of frequently asked questions and while I’m just a bot, there is a group of people who are happy to teach me new things. Let them know.

Good luck with your project ✨

You rock!

🌴

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The new version differs by 176 commits .

• a57565a 6.0.0
• 9de0461 Whitespace change.
• 4b36ac8 Prepare to release v6.0.0 (#544)
• afa9812 Remove the warning about no local node_modules install of wct. (#500)
• 08a5869 Merge pull request #542 from Polymer/polyserve
• fb39d57 update wct-local@^2.0.15 (#541)
• 96f7909 Fix tsc 2.3.0 warnings. (#543)
• 1aa04b5 update polyserver to 0.19.0
• 9ade01a bump version.
• 2877765 Merge pull request #537 from Polymer/wc-ready-check
• f533c18 remove extra spaces
• 712a66e Avoid relying on filename and instead use an explicit signal:
• 1bbe898 Check webcomponents script for "webComponentsReady" flag
• ad59963 6.0.0-prerelease.9
• 173b290 Run gulp build from npm run build; check in new browser.js.map

There are 176 commits in total. See the full diff.