dart2js: Simplify type test emitter.

pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
index aba512bd66dec10ea23f8da7eb0beac753ecdc3d..c926461bd958328d091dc79253510b5d1bb266ce 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
@@ -111,21 +111,19 @@ class TypeTestEmitter extends CodeEmitterHelper {
       }
       for (DartType supertype in cls.allSupertypes) {
         ClassElement superclass = supertype.element;
-        if (classesUsingTypeVariableTests.contains(superclass)) {
+        if (emitted.contains(superclass)) continue;
+
+        if (classesUsingTypeVariableTests.contains(superclass) ||
+            checkedClasses.contains(superclass)) {
+          // Generate substitution.  If no substitution is necessary, emit
+          // `null` to overwrite a (possibly) existing substitution from the
+          // super classes.
           emitSubstitution(superclass, emitNull: true);
-          emitted.add(superclass);
-        }
-        for (ClassElement check in checkedClasses) {
-          if (supertype.element == check && !emitted.contains(check)) {
-            // Generate substitution.  If no substitution is necessary, emit
-            // [:null:] to overwrite a (possibly) existing substitution from the
-            // super classes.
-            emitSubstitution(check, emitNull: true);
-            emitted.add(check);
-          }
         }
       }
+
       void emitNothing(_, {emitNull}) {};
+
       emitSubstitution = emitNothing;
     }