Polyserve tool re-writes correct JS generated by TS 3.7 to invalid synax when null coalesce & optional chaining is used

[lozandier]

When using TypeScript w/ Polyserve or PWA-starter-kit, the correct JS generated using TS 3.7 features get re-written in a weird way by the dev server pipelining leveraged by polymer serve:

foo?.someProp ?? "default value" 

gets re-written as invalid JS:

foo ? .someProp ? ? "default value : : :

The errors that populate involves babylon. Because of these errors, you have to stop the dev server constantly to re-run tsc to generate the correct code.

//cc @rictic

[stale]

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