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..0e7f012fcbcbcb2552bc3d955c89b52a11aadca5 100644
--- a/dart/frog/leg/ssa/codegen.dart
+++ b/dart/frog/leg/ssa/codegen.dart
@@ -929,6 +929,11 @@ 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.EQUALITY_PRECEDENCE);
+      buffer.add(' === (void 0) ||');
+      beginExpression(JSPrecedence.LOGICAL_OR_PRECEDENCE);

[Lasse Reichstein Nielsen, 2012/03/06 10:31:08]

Sadly, that won't do what you want. It doesn't change the expectations of the
following code, but merely puts a start parenthesis if the current expectation
is harder binding than a ||.

We can either create a function withPrecedence(precedence, thunk) that sets the
expected precedence during execution of  the thunk, or we can manually update
expectedPrecedence here (if it's the only place we need it, the extra
abstraction might be overdoing it).

I.e.:

int oldPrecedence = currentPrecedence;
if (node.nullOK) {
  ...
  currentPrecedence = LOGICAL_OR_PRECEDENCE;
}
... expression or RHS of || ...
if (node.nullOk) {
  currentPrecedence = oldPrecedence;
}

It breaks the abstraction a little, since we don't manipulate currentPrecedence
anywhere else outside of use/visit, but if it's only one place it should be
fine.

[ahe, 2012/03/06 10:57:38]

I think this is way too complicated, and I have no idea how to apply your
suggestion. The mental model behind this is apparently not based on building up
expressions from subexpressions. As long as that is not the case, this will be
an endless source of problems.

I think it was a mistake to add the precedence in this form. We would be better
off wrapping everything in parentheses.

I'll add a pair of parentheses.

+    }
     if (element == objectClass) {
       // TODO(ahe): This probably belongs in the constant folder.
       buffer.add('true');
@@ -994,6 +999,9 @@ class SsaCodeGenerator implements HVisitor {
       if (endParen) buffer.add(')');
       endExpression(JSPrecedence.LOGICAL_AND_PRECEDENCE);
     }
+    if (node.nullOk) {
+      endExpression(JSPrecedence.LOGICAL_OR_PRECEDENCE);
+    }
   }
 
   bool isStringSupertype(ClassElement cls) {