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

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 926 matching lines @@
 
   void checkNull(HInstruction input) {
     beginExpression(JSPrecedence.EQUALITY_PRECEDENCE);
     use(input, JSPrecedence.EQUALITY_PRECEDENCE);
     buffer.add(" === (void 0)");
     endExpression(JSPrecedence.EQUALITY_PRECEDENCE);
   }
 
   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;
     if (node.nullOk) {
       beginExpression(JSPrecedence.LOGICAL_OR_PRECEDENCE);
       checkNull(input);
       buffer.add(' || ');
     }
     if (element == objectClass) {
       // TODO(ahe): This probably belongs in the constant folder.
@@ skipping 40 matching lines @@
       use(input, JSPrecedence.MEMBER_PRECEDENCE);
       buffer.add('.');
       buffer.add(compiler.namer.operatorIs(node.typeExpression));
       if (element.isClass() && (element.dynamic.isNative()
                                 || isSupertypeOfNativeClass(element))) {
         buffer.add(' || ');
         beginExpression(JSPrecedence.LOGICAL_AND_PRECEDENCE);
         // 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('.');
         buffer.add(compiler.namer.operatorIs(objectClass));
         buffer.add(' && ');
         buffer.add(compiler.emitter.nativeEmitter.dynamicIsCheckName);
         buffer.add('(');
         use(input, JSPrecedence.MEMBER_PRECEDENCE);
         buffer.add(", '${compiler.namer.operatorIs(node.typeExpression)}')");
         endExpression(JSPrecedence.LOGICAL_AND_PRECEDENCE);
@@ skipping 386 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endElse(HIf node) {
     if (node.elseBlock.hasBailouts()) {
       endBailoutSwitch();
     }
   }
 }