Stop using deprecated LibraryElement.isNonNullableByDefault

Prepare for https://dart-review.googlesource.com/c/sdk/+/366700

PiperOrigin-RevId: 634787005

lib/src/builder.dart

@@ -74,7 +74,7 @@ class MockBuilder implements Builder {
   Future<void> build(BuildStep buildStep) async {
     if (!await buildStep.resolver.isLibrary(buildStep.inputId)) return;
     final entryLib = await buildStep.inputLibrary;
-    final sourceLibIsNonNullable = entryLib.isNonNullableByDefault;
+    final sourceLibIsNonNullable = true;
 
     final mockLibraryAsset = buildStep.allowedOutputs.singleOrNull;
     if (mockLibraryAsset == null) {
@@ -144,13 +144,11 @@ class MockBuilder implements Builder {
     // The source lib may be pre-null-safety because of an explicit opt-out
     // (`// @dart=2.9`), as opposed to living in a pre-null-safety package. To
     // allow for this situation, we must also add an opt-out comment here.
-    final dartVersionComment = sourceLibIsNonNullable ? '' : '// @dart=2.9';
     final mockLibraryContent = DartFormatter().format('''
 // Mocks generated by Mockito $packageVersion from annotations
 // in ${entryLib.definingCompilationUnit.source.uri.path}.
 // Do not manually edit this file.
 
-$dartVersionComment
 
 $rawOutput
 ''');
@@ -1051,7 +1049,7 @@ class _MockLibraryInfo {
       final fallbackGenerators = mockTarget.fallbackGenerators;
       mockClasses.add(_MockClassInfo(
         mockTarget: mockTarget,
-        sourceLibIsNonNullable: entryLib.isNonNullableByDefault,
+        sourceLibIsNonNullable: true,
         typeProvider: entryLib.typeProvider,
         typeSystem: entryLib.typeSystem,
         mockLibraryInfo: this,
@@ -1175,9 +1173,8 @@ class _MockClassInfo {
         // The test can be pre-null-safety but if the class
         // we want to mock is defined in a null safe library,
         // we still need to override methods to get nice mocks.
-        final isNiceMockOfNullSafeClass = mockTarget.onMissingStub ==
-                OnMissingStub.returnDefault &&
-            typeToMock.element.enclosingElement.library.isNonNullableByDefault;
+        final isNiceMockOfNullSafeClass =
+            mockTarget.onMissingStub == OnMissingStub.returnDefault;
 
         if (sourceLibIsNonNullable || isNiceMockOfNullSafeClass) {
           cBuilder.methods.addAll(