stuartmorgan b86b2a9bee Fix handler unregistration in C++ channels (flutter/engine#16794)
MethodChannel and BasicMessageChannel in the C++ wrapper didn't have the
expected semantics that passing a null handler would remove any existing
handler. This was inconsistent with other platforms and with the
lower-level object APIs in this wrapper, and made unregistration in
cases where that's desirable more difficult due to needing to keep other
object references.

Adds tests for this functionality, and some backfill of missing tests
for basic handler behavior.

See https://github.com/flutter/flutter/issues/51207
2020-02-25 14:09:58 -08:00
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