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

packages/elements/test/BUILD.bazel

@@ -30,6 +30,7 @@ filegroup(
     testonly = True,
     # do not sort
     srcs = [
+        "@npm//:node_modules/core-js/client/core.js",
         "@npm//:node_modules/@webcomponents/custom-elements/src/native-shim.js",
         "@npm//:node_modules/reflect-metadata/Reflect.js",
         "//packages/zone.js/dist:zone.js",
@@ -42,11 +43,6 @@ karma_web_test_suite(
     bootstrap = [
         ":elements_test_bootstrap_scripts",
     ],
-    tags = [
-        # FIXME: timed out in CI
-        "fixme-saucelabs-ivy",
-        "fixme-saucelabs-ve",
-    ],
     deps = [
         ":test_lib",
     ],