Deduplicate helper name strings in compiler #18863
Description
From @Andy-MS's feedback: https://github.com/Microsoft/TypeScript/pull/18300/files#r141959156
Let's make sure these helper names are contained in a singe place.
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From @Andy-MS's feedback: https://github.com/Microsoft/TypeScript/pull/18300/files#r141959156
Let's make sure these helper names are contained in a singe place.