Introduce WorldImpact

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 8b20d127545ec897fe061d01ee403f46463c72db..753faf24ca7976dfd3d45f8945776d20709bd502 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -55,7 +55,7 @@ abstract class TypeMask {
   factory TypeMask.nonNullExact(ClassElement base, ClassWorld classWorld) {
     assert(invariant(base, classWorld.isInstantiated(base),
         message: "Cannot create extact type mask for "
-                 "uninstantiated class $base"));
+                 "uninstantiated class $base."));
     return new FlatTypeMask.nonNullExact(base);
   }