Move nullabilitySuffix to the public interface for DartType.

Change-Id: Idb53afaf30312b6423b84baa988ec2bb9ca29c24
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125720
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>

pkg/analyzer/lib/dart/element/type.dart

@@ -20,6 +20,7 @@
 /// type parameters. But if we declare a variable as `Pair<String, int> pair;`
 /// the references to `String` and `int` are type arguments.
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/nullability_suffix.dart';
 import 'package:analyzer/src/dart/element/type.dart' show InterfaceTypeImpl;
 
 /// The type associated with elements in the element model.
@@ -111,6 +112,9 @@ abstract class DartType {
   /// such as when the type represents the type of an unnamed function.
   String get name;
 
+  /// Return the nullability suffix of this type.
+  NullabilitySuffix get nullabilitySuffix;
+
   /// If this type is a [TypeParameterType], returns its bound if it has one, or
   /// [objectType] otherwise.
   ///

pkg/analyzer/lib/src/dart/element/type.dart

@@ -2021,9 +2021,7 @@ abstract class TypeImpl implements DartType {
   @override
   bool get isVoid => false;
 
-  /**
-   * Return the nullability suffix of this type.
-   */
+  @override
   NullabilitySuffix get nullabilitySuffix;
 
   /**