Priority level of the diagnostic used to control which diagnostics should be shown and filtered.
Typically this only makes sense to set to a different value than
DiagnosticLevel.info for diagnostics representing properties. Some
subclasses have a
level argument to their constructor which influences
the value returned here but other factors also influence it. For example,
whether an exception is thrown computing a property value
DiagnosticLevel.error is returned.
DiagnosticLevel get level => DiagnosticLevel.info;