Elements. Compute class flags on ClassElementImpl

Move class-related flags off fragments and compute them directly on
`ClassElementImpl`. This reduces coupling to fragments and avoids
duplicated logic.

Changes:
- Compute `isConstructable` as `!isSealed && !isAbstract` instead of
  delegating to `_firstFragment`.
- Compute `isExhaustive` as `isSealed`.
- Remove `hasGenerativeConstConstructor`, `isConstructable`, and
  `isExhaustive` getters from `ClassFragmentImpl`.
- In constant verifier, resolve the enclosing class via
  `container.declaredFragment!.element` and check `ClassElementImpl`
  rather than fragment types.

Why:
- Centralizes flag computation on the element, making semantics clearer.
- Decreases fragment dependencies and maintenance surface.

Change-Id: I4b7153a4a55037e6dc12e18a32161a3472902eeb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446008
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/constant/constant_verifier.dart

@@ -1661,8 +1661,8 @@ extension on Expression {
               !declarationListParent.isStatic) {
             var container = declarationListParent.parent;
             if (container is ClassDeclaration) {
-              var enclosingClass = container.declaredFragment;
-              if (enclosingClass is ClassFragmentImpl) {
+              var enclosingClass = container.declaredFragment!.element;
+              if (enclosingClass is ClassElementImpl) {
                 // A field initializer of a class with at least one generative
                 // const constructor does not constitute a constant context, but
                 // must be a constant expression.

pkg/analyzer/lib/src/dart/element/element.dart

@@ -250,7 +250,7 @@ class ClassElementImpl extends InterfaceElementImpl implements ClassElement {
 
   @override
   @trackedIncludedInId
-  bool get isConstructable => _firstFragment.isConstructable;
+  bool get isConstructable => !isSealed && !isAbstract;
 
   @override
   @trackedIncludedInId
@@ -277,7 +277,7 @@ class ClassElementImpl extends InterfaceElementImpl implements ClassElement {
 
   @override
   @trackedIncludedInId
-  bool get isExhaustive => _firstFragment.isExhaustive;
+  bool get isExhaustive => isSealed;
 
   @override
   bool get isExtendableOutside => !isInterface && !isFinal && !isSealed;
@@ -559,15 +559,6 @@ class ClassFragmentImpl extends InterfaceFragmentImpl
   /// given [offset] in the file that contains the declaration of this element.
   ClassFragmentImpl({required super.name});
 
-  bool get hasGenerativeConstConstructor {
-    _ClassFragmentImplModifiers.hasExtendsClause;
-    return constructors.any((c) => !c.isFactory && c.isConst);
-  }
-
-  bool get isConstructable => !isSealed && !isAbstract;
-
-  bool get isExhaustive => isSealed;
-
   @override
   ClassFragmentImpl? get nextFragment {
     return super.nextFragment as ClassFragmentImpl?;