Reevaluate checkExternalEmitHelpers() callers

[saschanaz]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms: importHelpers

Code

Some callers still checks against ESNext for non-esnext stable features.

https://github.com/microsoft/TypeScript/blob/94b0e0e389f5328d1e909c4efd951dbd882829f8/src/compiler/checker.ts#L31351-L31353

Expected behavior: They should check against ES20xx target.

Actual behavior: They do against ESNext

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