crisbeto 191e4d15b5 build: fix elements test failures on IE (#35940)
Fixes the following issues which caused the `elements` unit tests to break on IE:
1. `core.js` wasn't included which caused an error about `Promise` and `Symbol` to be thrown.
2. We were using a version of `@webcomponents/custom-elements` which was shipping ES6 code to npm. As a result, IE was throwing a syntax error.

PR Close #35940
2020-03-10 14:16:34 -04:00
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