Collect the types used in is-checks in the resolver phase.

lib/compiler/implementation/js_backend/native_emitter.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 NativeEmitter {
 
   CodeEmitterTask emitter;
   CodeBuffer nativeBuffer;
 
   // Classes that participate in dynamic dispatch. These are the
@@ skipping 365 matching lines @@
   }
 
   bool requiresNativeIsCheck(Element element) {
     if (!element.isClass()) return false;
     ClassElement cls = element;
     if (cls.isNative()) return true;
     return isSupertypeOfNativeClass(element);
   }
 
   void emitIsChecks(Map<String, String> objectProperties) {
-    for (Element type in compiler.codegenWorld.isChecks) {
-      if (!requiresNativeIsCheck(type)) continue;
-      String name = compiler.namer.operatorIs(type);
+    for (Element element in compiler.codegenWorld.checkedClasses) {
+      if (!requiresNativeIsCheck(element)) continue;
+      String name = compiler.namer.operatorIs(element);
       objectProperties[name] = 'function() { return false; }';
     }
   }
 
   void assembleCode(CodeBuffer targetBuffer) {
     if (nativeClasses.isEmpty()) return;
     emitDynamicDispatchMetadata();
     targetBuffer.add('$defineNativeClassName = '
                      '$defineNativeClassFunction;\n\n');
 
@@ skipping 33 matching lines @@
         if (!first) targetBuffer.add(",\n");
         targetBuffer.add(" $name: $function");
         first = false;
       });
       targetBuffer.add("\n});\n\n");
     }
     targetBuffer.add('$nativeBuffer');
     targetBuffer.add('\n');
   }
 }