Revert "dart2js cps: Add CharCodeAt builtin."

pkg/compiler/lib/src/types/type_mask.dart

Index: pkg/compiler/lib/src/types/type_mask.dart
diff --git a/pkg/compiler/lib/src/types/type_mask.dart b/pkg/compiler/lib/src/types/type_mask.dart
index 6dab2eace97fac00f60f923a39ff9d552a905caa..c255b6040d2fabecfe9e5e58cb8abe2226e04f6e 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -145,9 +145,9 @@ abstract class TypeMask implements ReceiverConstraint {
   }
 
   factory TypeMask.nonNullSubclass(ClassElement base, ClassWorld classWorld) {
-    //assert(invariant(base, classWorld.isInstantiated(base),
-    //    message: () => "Cannot create subclass type mask for uninstantiated "
-    //                   "class $base.\n${classWorld.dump(base)}"));
+    assert(invariant(base, classWorld.isInstantiated(base),
+        message: () => "Cannot create subclass type mask for uninstantiated "
+                       "class $base.\n${classWorld.dump(base)}"));
     ClassElement topmost = classWorld.getLubOfInstantiatedSubclasses(base);
     if (topmost == null) {
       return new TypeMask.nonNullEmpty();