For reasons beyond logic, null matches any exception type.

dart/frog/leg/ssa/codegen.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   SsaCodeGeneratorTask(Compiler compiler) : super(compiler);
   String get name() => 'SSA code generator';
 
   String generate(WorkItem work, HGraph graph) {
     return measure(() {
@@ skipping 911 matching lines @@
     use(input, JSPrecedence.MEMBER_PRECEDENCE);
     buffer.add('.constructor $cmp Array');
     endExpression(JSPrecedence.EQUALITY_PRECEDENCE);
   }
 
   void visitIs(HIs node) {
     ClassElement element = node.typeExpression;
     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');
     } else if (element == coreLibrary.find(const SourceString('String'))) {
       checkString(input, '===');
     } else if (element == coreLibrary.find(const SourceString('double'))) {
       checkDouble(input, '===');
     } else if (element == coreLibrary.find(const SourceString('num'))) {
       checkNum(input, '===');
     } else if (element == coreLibrary.find(const SourceString('bool'))) {
@@ skipping 433 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endElse(HIf node) {
     if (node.elseBlock.hasBailouts()) {
       endBailoutSwitch();
     }
   }
 }