Wp-calypso: Block-Editor: Missing DOMRect polyfill for IE 11

[gwwar]

@ockham @gwwar and @kwight were investigating E2E failures and noticed that we're missing the DOMRect polyfill in IE11.

This will cause several JS errors to fire as folks interact with a block toolbar in the block-editor when using IE11

• [x] Please add back the polyfill as a quickfix similar to D39800-code (D40956-code)
• [x] File an issue upstream to Gutenberg core project since this also appears to be missing in Gutenberg 7.8 (https://github.com/WordPress/gutenberg/issues/21185)
• [x] Add back block editor E2E test to IE11 canaries (removed in #40510 to unblock PRs) (#40517)

Related: https://github.com/WordPress/gutenberg/pull/20110

[kwight]

I did notice that there's no vendor directory in the v7.7.1 dotcom Gutenberg plugin, so it may be a related to the plugin build process before the plugin was uploaded to dotcom.

[simison]

Thanks! Logged also https://github.com/Automattic/wp-calypso/issues/40468

[mmtr]

In order to unblock other PRs, I removed the failing test from the IE11 canaries in https://github.com/Automattic/wp-calypso/pull/40510 while we fix this.

Please add it back once the polyfill has been included.

[p-jackson]

I've added the block editor E2E tests back to IE11 canaries. This involved making the tests work regardless of whether it was using the wp-admin or iframe editor, details in #40517