For reasons beyond logic, null matches any exception type.

dart/frog/leg/ssa/codegen.dart

Index: dart/frog/leg/ssa/codegen.dart
diff --git a/dart/frog/leg/ssa/codegen.dart b/dart/frog/leg/ssa/codegen.dart
index 7921553b4d9fb8a9449943a6aa370a594a6e9e35..46b705d5e7a4c11cd9bf63c57b29dd14523e2ad4 100644
--- a/dart/frog/leg/ssa/codegen.dart
+++ b/dart/frog/leg/ssa/codegen.dart
@@ -929,6 +929,10 @@ class SsaCodeGenerator implements HVisitor {
     LibraryElement coreLibrary = compiler.coreLibrary;
     ClassElement objectClass = coreLibrary.find(const SourceString('Object'));
     HInstruction input = node.expression;
+    if (node.nullOk) {
+      use(input, JSPrecedence.LOGICAL_OR_PRECEDENCE);

[Lasse Reichstein Nielsen, 2012/03/01 08:38:17]

That should be EQUALITY_PRECEDENCE. It's the left hand side of an equality
comparison.

[ahe, 2012/03/06 10:16:01]

Done.

+      buffer.add(' === (void 0) ||');

[ahe, 2012/02/29 20:44:20]

I think this will work well for checked mode as well.

+    }

[Lasse Reichstein Nielsen, 2012/03/02 09:44:09]

This also changes the expected precedence for the following code.
The check* methods are safe, since they generate === operators, but the && and
|| expressions below uses beginExpression relative to the outer precedence, not
the precedence of the || inserted above. 
I think we need some support for executing code in a different precedence scope.

[ahe, 2012/03/06 10:16:01]

Done.

     if (element == objectClass) {
       // TODO(ahe): This probably belongs in the constant folder.
       buffer.add('true');