Collect is checks to only emit the ones that are needed.

frog/leg/ssa/codegen.dart

Index: frog/leg/ssa/codegen.dart
===================================================================
--- frog/leg/ssa/codegen.dart	(revision 5280)
+++ frog/leg/ssa/codegen.dart	(working copy)
@@ -944,6 +944,7 @@
 
   void visitIs(HIs node) {
     Element element = node.typeExpression;
+    compiler.registerIsCheck(element);
     LibraryElement coreLibrary = compiler.coreLibrary;
     ClassElement objectClass = coreLibrary.find(const SourceString('Object'));
     HInstruction input = node.expression;
@@ -1004,6 +1005,7 @@
         // First check if the object is not a Dart object. If the
         // object is a Dart object, we know the property check was
         // sufficient.
+        compiler.registerIsCheck(objectClass);
         buffer.add('!');
         use(input, JSPrecedence.MEMBER_PRECEDENCE);
         buffer.add('.');