dart2js crashes on nullable bools in checked mode

The following code throws an error when run with enable_type_checks and enable_asserts under frog:

&nbsp;&nbsp;bool foo(a) =&gt; a;

&nbsp;&nbsp;main() {
&nbsp;&nbsp;&nbsp;&nbsp;print(foo(null));
&nbsp;&nbsp;}

This is unexpected, since Dart allows null as a value of bool.
