[analyzer] Make two derived classes for PropertyAccessorElementImpl.

Derived class `GetterFragmentImpl` is used for getters and
`SetterFragmentImpl` is used for setters. Only `GetterFragmentImpl`
implements `GetterFragment` and only `SetterFragmentImpl` implements
`SetterFragment`. This ensures that clients performing `is
GetterFragment` and `is SetterFragment` checks get the expected
behavior. (Previously, any instance of `PropertyAccessorElementImpl`
would satisfy *both* `is GetterFragment` and `is SetterFragment`
tests).

This made it possible for the types of getters defined in
`GetterFragment` and `SetterFragment` to be more precise, which in
turn allowed some casts to be removed from the analyzer.

Change-Id: I243e8f6547d9aed77175e8c14c48af8970bfd528
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/412140
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analysis_server/lib/src/services/refactoring/legacy/convert_getter_to_method.dart

@@ -105,7 +105,7 @@ class ConvertGetterToMethodRefactoringImpl extends RefactoringImpl
     for (
       GetterFragment? fragment = element.firstFragment;
       fragment != null;
-      fragment = fragment.nextFragment as GetterFragment?
+      fragment = fragment.nextFragment
     ) {
       var nameRange = range.fragmentName(fragment);
       if (nameRange == null) {

pkg/analyzer/api.txt

@@ -3873,6 +3873,9 @@ package:analyzer/dart/element/element2.dart:
   GetterFragment (class extends Object implements PropertyAccessorFragment):
     new (constructor: GetterFragment Function())
     correspondingSetter2 (getter: SetterFragment?)
+    element (getter: GetterElement)
+    nextFragment (getter: GetterFragment?)
+    previousFragment (getter: GetterFragment?)
   HasSinceSdkVersion (class extends Object):
     new (constructor: HasSinceSdkVersion Function())
     sinceSdkVersion (getter: Version?)
@@ -4203,6 +4206,9 @@ package:analyzer/dart/element/element2.dart:
   SetterFragment (class extends Object implements PropertyAccessorFragment):
     new (constructor: SetterFragment Function())
     correspondingGetter2 (getter: GetterFragment?)
+    element (getter: SetterElement)
+    nextFragment (getter: SetterFragment?)
+    previousFragment (getter: SetterFragment?)
   ShowElementCombinator (see above)
   SuperFormalParameterElement2 (class extends Object implements FormalParameterElement):
     new (constructor: SuperFormalParameterElement2 Function())

pkg/analyzer/lib/dart/element/element2.dart

@@ -1181,22 +1181,14 @@ abstract class GetterFragment implements PropertyAccessorFragment {
   /// `null` if there is no corresponding setter.
   SetterFragment? get correspondingSetter2;
 
-  // TODO(brianwilkerson): This should override `nextFragment` to be more
-  //  specific, but can't because the Impl class supports both getters and
-  //  setters.
-  // @override
-  // GetterFragment? get nextFragment;
+  @override
+  GetterElement get element;
 
-  // TODO(brianwilkerson): This should override `previousFragment` to be more
-  //  specific, but can't because the Impl class supports both getters and
-  //  setters.
-  // @override
-  // GetterFragment? get previousFragment;
+  @override
+  GetterFragment? get nextFragment;
 
-  // TODO(brianwilkerson): This should override `element` to be more specific,
-  //  but can't because the Impl class supports both getters and setters.
-  // @override
-  // GetterElement get element;
+  @override
+  GetterFragment? get previousFragment;
 }
 
 /// The interface that is implemented by elements that can have `@Since()`
@@ -2502,22 +2494,14 @@ abstract class SetterFragment implements PropertyAccessorFragment {
   /// `null` if there is no corresponding getter.
   GetterFragment? get correspondingGetter2;
 
-  // TODO(brianwilkerson): This should override `element` to be more specific,
-  //  but can't because the Impl class supports both getters and setters.
-  // @override
-  // SetterElement get element;
+  @override
+  SetterElement get element;
 
-  // TODO(brianwilkerson): This should override `nextFragment` to be more
-  //  specific, but can't because the Impl class supports both getters and
-  //  setters.
-  // @override
-  // SetterFragment? get nextFragment;
+  @override
+  SetterFragment? get nextFragment;
 
-  // TODO(brianwilkerson): This should override `previousFragment` to be more
-  //  specific, but can't because the Impl class supports both getters and
-  //  setters.
-  // @override
-  // SetterFragment? get previousFragment;
+  @override
+  SetterFragment? get previousFragment;
 }
 
 /// A super formal parameter.