Don't write runtimeType as test expectation.

Author: scheglov

source_gen/test/generator_for_annotation_test.dart

@@ -161,7 +161,18 @@ $dartFormatWidth
     final builder = LibraryBuilder(
       _StubGenerator<Deprecated>(
         'Deprecated',
-        directiveBehavior: (element) => '// ${element.runtimeType}',
+        directiveBehavior: (element) {
+          switch (element) {
+            case LibraryImport():
+              return '// LibraryImport';
+            case LibraryExport():
+              return '// LibraryExport';
+            case PartInclude():
+              return '// PartInclude';
+            default:
+              throw UnimplementedError('${element.runtimeType}');
+          }
+        },
         elementBehavior: (element) => '// ${element.runtimeType}',
       ),
     );
@@ -189,11 +200,11 @@ $dartFormatWidth
 // Generator: Deprecated
 // **************************************************************************
 
-// LibraryImportElementImpl
+// LibraryImport
 
-// LibraryExportElementImpl
+// LibraryExport
 
-// PartElementImpl
+// PartInclude
 ''',
       },
     );