APIs in dart:html need to be fixed based on results from native null assertions

[srujzs]

For context, #42536 and #42535 added a flag to both dart2js and ddc to check that dart:html and similar libraries' native APIs are sound. Currently, they are being tested internally in google3 to ensure their correctness. There are several observed instances where a native API returns a nullable (either due to the API being undefined/incompatible or because it actually returns null), but is typed as non-nullable. dart:html needs to be fixed to account for this discrepancy. This bug tracks work around those fixes.

Similarly, as libraries migrate to sound null-safety they may need to work around this limitation by casting the return values from these APIs as nullable even though they are actually non-nullable. When those APIs are fixed, those workarounds should be removed.