Dart does not throw error if class argument parameter extends Null

[iarkh]

Dart SDK Version: 2.0.0-dev.55.0
OS: Windows 10

Sample code below declares a class with parameter which extends [Null]. It causes compiler error with dartanalyzer and passes without errors with dart.

class A<X extends Null> {}
main() {}

This is similar to the issues #33699, #33701.
I believe both tools should behave in the same way.
Dartanalyzer sample output is:

Analyzing test.dart...
  error - 'X' can't be a supertype of its upper bound at test.dart:1:9 - type_parameter_supertype_of_its_bound
1 error found.

Dart stdout and stderr are empty, no exceptions or warnings there.

[eernstg]

The discussion on #33701 revealed that the analyzer behaves as specified, and we need to decide (1) change the specification and the analyzer, or (2) just note that this is already working as intended in the analyzer.

Created issue #33709 in order to clarify the language question.

Marking this as blocked on #33709, and setting the label to area-front-end, which is the correct choice for the specification of today.