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

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 736df0d614f3ef32e2e76d995e4648cb5fdbc9a2..0fdd5792db34761368678d42f04a347772464d66 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -247,8 +247,20 @@ abstract class TypeMask implements ReceiverConstraint, AbstractValue {
    */
   TypeMask nonNullable();
 
+  /// Whether nothing matches this mask, not even null.
   bool get isEmpty;
+
+  /// Whether null is a valid value of this mask.
   bool get isNullable;
+
+  /// Whether the only possible value in this mask is Null.
+  bool get isNull;
+
+  /// Whether [isEmpty] or [isNull] is true.
+  bool get isEmptyOrNull;
+
+  /// Whether this mask only includes instances of an exact class, and none of
+  /// it's subclasses or subtypes.
   bool get isExact;
 
   /// Returns true if this mask is a union type.