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

frog/leg/universe.dart

Index: frog/leg/universe.dart
===================================================================
--- frog/leg/universe.dart	(revision 5280)
+++ frog/leg/universe.dart	(working copy)
@@ -11,6 +11,8 @@
   final Set<SourceString> invokedGetters;
   final Set<SourceString> invokedSetters;
   final Map<String, LibraryElement> libraries;
+  // TODO(ngeoffray): This should be a Set<Type>.
+  final Set<ClassElement> isChecks;
 
   Universe() : generatedCode = new Map<Element, String>(),
                generatedBailoutCode = new Map<Element, String>(),
@@ -19,7 +21,8 @@
                staticFunctionsNeedingGetter = new Set<FunctionElement>(),
                invokedNames = new Map<SourceString, Set<Invocation>>(),
                invokedGetters = new Set<SourceString>(),
-               invokedSetters = new Set<SourceString>();
+               invokedSetters = new Set<SourceString>(),
+               isChecks = new Set<ClassElement>();
 
   void addGeneratedCode(WorkItem work, String code) {
     if (work.isBailoutVersion()) {