Elements. Fix method top-merge reuse and default constructor flag

Correct two issues in element handling:

* In `InheritanceManager3._topMerge`, when merging methods, check for
  an existing `MethodElementImpl` at `@method/<name>` (not a setter).
  If present, reuse it; otherwise assert the slot is empty before
  synthesizing. Similar assertions are added to getter/setter paths to
  guard against accidental reuse of a wrong-kind element.

* In `SubstitutedConstructorElementImpl`, fix
  `isDefaultConstructor` to forward to
  `baseElement.isDefaultConstructor` (it previously forwarded to
  `isConst`).

These changes prevent creating fresh synthetic methods when a correct
method already exists, preserving element identity and avoiding
wrong-kind lookups. The constructor fix restores accurate default
constructor detection, which can affect downstream diagnostics and
presentation.

Additional tests cover method top-merge in both “existing” and
“synthetic” scenarios, aligning method coverage with existing
getter/setter cases.

Change-Id: I09c3aaa7cb89f216d6d3101bbad835567df5b213
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447961
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/element/inheritance_manager3.dart

@@ -1069,9 +1069,10 @@ class InheritanceManager3 {
       var elementReference = targetClass.reference!
           .getChild('@method')
           .getChild(fragmentName);
-      if (elementReference.element case SetterElementImpl result) {
+      if (elementReference.element case MethodElementImpl result) {
         return result;
       }
+      assert(elementReference.element == null);
 
       var resultFragment = MethodFragmentImpl(name: fragmentName);
       resultFragment.enclosingFragment = targetClass.firstFragment;
@@ -1106,6 +1107,7 @@ class InheritanceManager3 {
         if (elementReference.element case GetterElementImpl result) {
           return result;
         }
+        assert(elementReference.element == null);
 
         var fragment = GetterFragmentImpl(name: fragmentName);
         resultFragment = fragment;
@@ -1120,6 +1122,7 @@ class InheritanceManager3 {
         if (elementReference.element case SetterElementImpl result) {
           return result;
         }
+        assert(elementReference.element == null);
 
         var fragment = SetterFragmentImpl(name: fragmentName);
         resultFragment = fragment;

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

@@ -61,7 +61,7 @@ class SubstitutedConstructorElementImpl extends SubstitutedExecutableElementImpl
   bool get isConst => baseElement.isConst;
 
   @override
-  bool get isDefaultConstructor => baseElement.isConst;
+  bool get isDefaultConstructor => baseElement.isDefaultConstructor;
 
   @override
   bool get isFactory => baseElement.isFactory;

pkg/analyzer/test/src/dart/element/inheritance_manager3_test.dart

@@ -1216,7 +1216,27 @@ abstract class X extends A implements B {}
     );
   }
 
-  test_getMember_optIn_topMerge_method() async {
+  test_getMember_optIn_topMerge_method_existing() async {
+    await resolveTestCode('''
+class A {
+  dynamic foo() {}
+}
+
+class B {
+  Object? foo() {}
+}
+
+class X extends A implements B {}
+''');
+
+    _assertGetMember(
+      className: 'X',
+      name: 'foo',
+      expected: 'B.foo: Object? Function()',
+    );
+  }
+
+  test_getMember_optIn_topMerge_method_synthetic() async {
     await resolveTestCode('''
 class A {
   Object? foo(dynamic x) {}