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

lib/compiler/implementation/js_backend/emitter.dart

Index: lib/compiler/implementation/js_backend/emitter.dart
diff --git a/lib/compiler/implementation/js_backend/emitter.dart b/lib/compiler/implementation/js_backend/emitter.dart
index 0f64e081763a87415a4b77a3064801fa1bffefa7..938ee46045235862dcda565aa13ce72d595fa1b3 100644
--- a/lib/compiler/implementation/js_backend/emitter.dart
+++ b/lib/compiler/implementation/js_backend/emitter.dart
@@ -451,7 +451,7 @@ function(collectedClasses) {
   }
 
   String generateCheckedSetter(Element member, String fieldName) {
-    Type type = member.computeType(compiler);
+    DartType type = member.computeType(compiler);
     if (type.element.isTypeVariable()
         || type.element == compiler.dynamicClass
         || type.element == compiler.objectClass) {
@@ -622,7 +622,7 @@ function(collectedClasses) {
 
   void generateTypeTests(ClassElement cls,
                          void generateTypeTest(ClassElement element)) {
-    if (compiler.codegenWorld.isChecks.contains(cls)) {
+    if (compiler.codegenWorld.checkedClasses.contains(cls)) {
       generateTypeTest(cls);
     }
     generateInterfacesIsTests(cls, generateTypeTest, new Set<Element>());
@@ -634,7 +634,7 @@ function(collectedClasses) {
     for (DartType interfaceType in cls.interfaces) {
       Element element = interfaceType.element;
       if (!alreadyGenerated.contains(element) &&
-          compiler.codegenWorld.isChecks.contains(element)) {
+          compiler.codegenWorld.checkedClasses.contains(element)) {
         alreadyGenerated.add(element);
         generateTypeTest(element);
       }