Update constants_test.dart based on new constant evaluator changes. (#677)

* Update constants_test.dart

Fix the constants test now that the constant evaluator will have new changes and don't resolve with these errors in the test code.

Changes found: https://dart-review.googlesource.com/c/sdk/+/312347

* Eager version bump

---------

Co-authored-by: Nate Bosch <nbosch@google.com>

Author: kallentu

source_gen/CHANGELOG.md

@@ -1,3 +1,5 @@
+## 1.4.1-wip
+
 ## 1.4.0
 
 - Require Dart 3.0

source_gen/pubspec.yaml

@@ -1,5 +1,5 @@
 name: source_gen
-version: 1.4.0
+version: 1.4.1-wip
 description: >-
   Source code generation builders and utilities for the Dart build system
 repository: https://github.com/dart-lang/source_gen/tree/master/source_gen

source_gen/test/constants_test.dart

@@ -35,7 +35,6 @@ void main() {
           aString: aString,
           aInt: aInt,
           aBool: aBool,
-          aNull: aNull,
           nested: const Example(),
         )
         @Super()    // [5]
@@ -47,10 +46,10 @@ void main() {
         @aType      // [11]
         @aSet       // [12]
         class Example {
-          final String aString;
-          final int aInt;
-          final bool aBool;
-          final Example nested;
+          final String? aString;
+          final int? aInt;
+          final bool? aBool;
+          final Example? nested;
 
           const Example({this.aString, this.aInt, this.aBool, this.nested});
         }