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

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

Index: pkg/compiler/lib/src/types/value_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/value_type_mask.dart b/pkg/compiler/lib/src/types/value_type_mask.dart
index 72c5af28e7a4ca1192da5fb7a19b21c71cd40e7f..ffe1e3bc64f7ece9cca14d1caf4973b22dd0c8d2 100644
--- a/pkg/compiler/lib/src/types/value_type_mask.dart
+++ b/pkg/compiler/lib/src/types/value_type_mask.dart
@@ -31,7 +31,7 @@ class ValueTypeMask extends ForwardingTypeMask {
 
   TypeMask intersection(TypeMask other, ClassWorld classWorld) {
     TypeMask forwardIntersection = forwardTo.intersection(other, classWorld);
-    if (forwardIntersection.isEmpty) return forwardIntersection;
+    if (forwardIntersection.isEmptyOrNull) return forwardIntersection;
     return forwardIntersection.isNullable
         ? nullable()
         : nonNullable();
@@ -46,4 +46,4 @@ class ValueTypeMask extends ForwardingTypeMask {
   String toString() {
     return 'Value mask: [${value.unparse()}] type: $forwardTo';
   }
-}
+}