Elements. Make InterfaceElementImpl.session AnalysisSessionImpl.

Strengthen the `session` type on `InterfaceElementImpl` from nullable
`AnalysisSession?` to concrete `AnalysisSessionImpl` and route it
through `library.session`. This reflects the actual runtime type,
eliminates downcasts, and makes the API non-nullable.

Key changes:
- `InterfaceElementImpl.session` now returns `AnalysisSessionImpl`.
- Replaced usages of `(session as AnalysisSessionImpl)` with direct `session.inheritanceManager...` calls (e.g., inherited/interface member lookups and overrides).
- Removed a generic nullable `session` getter that delegated to an enclosing fragment, and dropped redundant fragment-level overrides where the library already provides the session.

Why:
- Improves type safety and clarity by matching the concrete session type.
- Removes unnecessary nullability and casting boilerplate.
- Centralizes the source of truth for the session via the owning library.

Change-Id: I2f8e045c34218e9e10caa794b830219326c7da1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446006
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: scheglov

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

@@ -3393,11 +3393,6 @@ abstract class FragmentImpl with _FragmentImplMixin implements Fragment {
   @Deprecated('Use nameOffset instead')
   int? get nameOffset2 => nameOffset;
 
-  /// The analysis session in which this element is defined.
-  AnalysisSession? get session {
-    return enclosingFragment?.session;
-  }
-
   /// The version where this SDK API was added.
   ///
   /// A `@Since()` annotation can be applied to a library declaration,
@@ -4027,7 +4022,7 @@ abstract class InstanceElementImpl extends ElementImpl
   Element get nonSynthetic => isSynthetic ? enclosingElement : this as Element;
 
   @override
-  AnalysisSession? get session => _firstFragment.session;
+  AnalysisSessionImpl get session => library.session;
 
   @override
   List<SetterElementImpl> get setters {
@@ -4621,18 +4616,15 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
 
   @override
   Map<Name, ExecutableElement> get inheritedConcreteMembers =>
-      (session as AnalysisSessionImpl).inheritanceManager
-          .getInheritedConcreteMap(this);
+      session.inheritanceManager.getInheritedConcreteMap(this);
 
   @override
   Map<Name, ExecutableElement> get inheritedMembers =>
-      (session as AnalysisSessionImpl).inheritanceManager.getInheritedMap(this);
+      session.inheritanceManager.getInheritedMap(this);
 
   @override
   Map<Name, ExecutableElement> get interfaceMembers =>
-      (session as AnalysisSessionImpl).inheritanceManager
-          .getInterface(this)
-          .map;
+      session.inheritanceManager.getInterface(this).map;
 
   @override
   @trackedIncludedInId
@@ -4737,14 +4729,11 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
 
   @override
   ExecutableElement? getInheritedMember(Name name) =>
-      (session as AnalysisSessionImpl).inheritanceManager.getInherited(
-        this,
-        name,
-      );
+      session.inheritanceManager.getInherited(this, name);
 
   @override
   ExecutableElement? getInterfaceMember(Name name) =>
-      (session as AnalysisSessionImpl).inheritanceManager.getMember(this, name);
+      session.inheritanceManager.getMember(this, name);
 
   @override
   ConstructorElementImpl? getNamedConstructor(String name) {
@@ -4766,10 +4755,7 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
 
   @override
   List<ExecutableElement>? getOverridden(Name name) =>
-      (session as AnalysisSessionImpl).inheritanceManager.getOverridden(
-        this,
-        name,
-      );
+      session.inheritanceManager.getOverridden(this, name);
 
   @override
   InterfaceTypeImpl instantiate({
@@ -6447,9 +6433,6 @@ class LibraryFragmentImpl extends FragmentImpl
     return _scope ??= LibraryFragmentScope(this);
   }
 
-  @override
-  AnalysisSession get session => library.session;
-
   @override
   List<SetterFragmentImpl> get setters => _setters;
 
@@ -7904,9 +7887,6 @@ class MultiplyDefinedElementImpl extends ElementImpl
   @override
   Element get nonSynthetic => this;
 
-  @override
-  AnalysisSession get session => libraryFragment.session;
-
   @override
   T? accept<T>(ElementVisitor2<T> visitor) {
     return visitor.visitMultiplyDefinedElement(this);