Migrate const_finder tests to be null safe (#37683)

* Bump language version of package:const_finder_fixtures in preparation of
running with sound null safety be default.

* Fix null safety errors.

* Migrate test files to be null safe.

* Fix analyzer warnings.

Author: a-siva

tools/const_finder/test/fixtures/.dart_tool/package_config.json

@@ -4,12 +4,12 @@
         {
             "name": "const_finder_fixtures",
             "rootUri": "../lib/",
-            "languageVersion": "2.9"
+            "languageVersion": "2.12"
         },
         {
             "name": "const_finder_fixtures_package",
             "rootUri": "../pkg/",
-            "languageVersion": "2.9"
+            "languageVersion": "2.12"
         }
     ]
 }

tools/const_finder/test/fixtures/lib/box.dart

@@ -10,8 +10,8 @@
 
 class Box {
   const Box(this.content1, this.content2);
-  final Object content1;
-  final Object content2;
+  final Object? content1;
+  final Object? content2;
 }
 
 const Box box1_0 = Box(null, null);

tools/const_finder/test/fixtures/lib/consts.dart

@@ -40,7 +40,7 @@ void main() {
 }
 
 class IgnoreMe {
-  const IgnoreMe([this.target]);
+  const IgnoreMe(this.target);
 
   final Target target;
 
@@ -68,7 +68,7 @@ class StaticConstInitializer {
   void useOne(int index) {
     targets[index].hit();
     targetSet.skip(index).first.hit();
-    targetMap[index].hit();
+    targetMap[index]!.hit();
   }
 }
 

tools/const_finder/test/fixtures/lib/consts_and_non.dart

@@ -31,7 +31,7 @@ void main() {
 }
 
 class IgnoreMe {
-  const IgnoreMe([this.target]);
+  const IgnoreMe(this.target);
 
   final Target target;
 

tools/const_finder/test/fixtures/lib/target.dart

@@ -7,15 +7,15 @@ class Target {
 
   final String stringValue;
   final int intValue;
-  final Target targetValue;
+  final Target? targetValue;
 
   void hit() {
     print('$stringValue $intValue');
   }
 }
 
 class ExtendsTarget extends Target {
-  const ExtendsTarget(String stringValue, int intValue, Target targetValue)
+  const ExtendsTarget(String stringValue, int intValue, Target? targetValue)
       : super(stringValue, intValue, targetValue);
 }
 
@@ -27,7 +27,7 @@ class ImplementsTarget implements Target {
   @override
   final int intValue;
   @override
-  final Target targetValue;
+  final Target? targetValue;
 
   @override
   void hit() {