For reasons beyond logic, null matches any exception type.

dart/frog/leg/ssa/builder.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 Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 2280 matching lines @@
         VariableDefinitions declaration = catchBlock.formals.nodes.head;
         HInstruction condition = null;
         if (declaration.type == null) {
           condition = graph.addNewLiteralTrue();
           stack.add(condition);
         } else {
           Element typeElement = elements[declaration.type];
           if (typeElement == null) {
             compiler.cancel('Catch with unresolved type', node: catchBlock);
           }
-          condition = new HIs(typeElement, unwrappedException);
+          condition = new HIs(typeElement, unwrappedException, nullOk: true);
           push(condition);
         }
       }
 
       void visitThen() {
         CatchBlock catchBlock = link.head;
         link = link.tail;
         VariableDefinitions declaration = catchBlock.formals.nodes.head;
         localsHandler.updateLocal(elements[declaration.definitions.nodes.head],
                                   unwrappedException);
@@ skipping 54 matching lines @@
     // Normally, we would call [close] here. However, then we hit
     // another unimplemented feature: aborting loop body. Simply
     // calling [add] does not work as it asserts that the instruction
     // isn't a control flow instruction. So we inline parts of [add].
     current.addAfter(current.last, new HThrow(message));
     if (isExpression) {
       stack.add(graph.addNewLiteralNull());
     }
   }
 }