Prepare for separation of ClassElement, EnumElement and MixinElement. (#617)

* Prepare for separation of ClassElement, EnumElement and MixinElement.

* Throw explicitly.

Author: scheglov

source_gen/lib/src/constants/revive.dart

@@ -44,7 +44,7 @@ Revivable reviveInstance(DartObject object, [LibraryElement? origin]) {
     );
   }
 
-  if (element is ClassElement) {
+  if (element is InterfaceElement) {
     for (final e in element.fields.where(
       (f) => f.isPublic && f.isConst && f.computeConstantValue() == object,
     )) {

source_gen/test/generator_for_annotation_test.dart

@@ -197,7 +197,7 @@ class _TestingResolver implements ReleasableResolver {
     bool allowSyntaxErrors = false,
   }) async {
     resolvedLibs.add(assetId);
-    return null as LibraryElement;
+    throw StateError('This method intentionally throws');
   }
 
   @override