[messages] Fix non_const_argument_for_const_parameter on RHS of a compound assignment.

Previously, if a compound assignment made use of a user-defined
operator whose argument was annotated `@mustBeConst`, the analyzer
would attempt to issue a `non_const_argument_for_const_parameter`
warning, regardless of whether the argument was indeed const. The
justification, from comments in the analyzer code, was that "the
argument cannot be const, as it depends on the value of the left hand
side", but this is not the case: in a compound assignment, the
argument to the user-defined operator is the right hand side, so it
does _not_ depend on the value of the left hand side.

To make matters worse, the code for issuing the warning tried to use
the AST node of the right hand side as the message parameter, whereas
the message required a string (the _name_ of the parameter), leading
to a crash. The reason this was not detected previously was that the
code path in question wasn't covered by tests.

I've fixed the logic so that the warning only fires if the right hand
side is not const. The fix involves calling the `_check` method (which
passes the appropriate value for the message parameter), so this also
fixes the crash. I've fixed the etst coverage problem by adding the
appropriate unit tests.

Fixes https://github.com/dart-lang/sdk/issues/62446.

Change-Id: I6a6a69649abaf7f111d2042053064ef76541657d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/474381
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>