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A lot of JS inline function calls were being passed null instead of nullRef. Extension conversions are changed to better handle WasmExternRef?s. There were a few spots in the dart:js_interop_unsafe API surface as well that were not handling nulls consistently. CoreLibraryReviewExempt: Doc changes to backend-specific lib. Change-Id: I08c5020e65156faed3841b0446aafb1e51242342 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/334380 Reviewed-by: Aske Simon Christensen <askesc@google.com>