Replace TypeMask.isEmpty by isEmptyOrNull, use isEmpty when it's really empty.

pkg/compiler/lib/src/cps_ir/type_mask_system.dart

Index: pkg/compiler/lib/src/cps_ir/type_mask_system.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
index 2042320f6fced3778e14b6c388c45cfde57b331c..b7a070e3b37798efc295de362989ac6fe78b5f4f 100644
--- a/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_mask_system.dart
@@ -281,7 +281,7 @@ class TypeMaskSystem implements AbstractValueDomain {
       TypeMask computed = computeTypeMask(inferrer.compiler, constant);
       TypeMask inferred = inferrer.getGuaranteedTypeOfElement(element);
       TypeMask best = intersection(inferred, computed);
-      assert(!best.isEmpty);
+      assert(!best.isEmptyOrNull);
       _constantMasks[constant] = best;
     }
   }