[analyzer] Use Impl types in TypeProviderImpl.

Change the methods in `TypeProviderImpl` so that their return types
are all "Impl" types rather than analyzer public API types.

This doesn't change the analyzer public API; it simply ensures that
clients of `TypeProviderImpl` within the analyzer itself won't have to
perform typecasts on the returned values in order to access the Impl
classes.

This is part of a larger arc of work to change the analyzer's use of
the shared code so that the type parameters it supplies are not part
of the analyzer public API (See
#59763). This change will
reduce the number of type casts that have to be done in order to pass
`TypeImpl` types to shared code rather than `DartType` types.

Change-Id: Ia351c3e8893cf10671b04b813ee80ff2b3bda341
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/402621
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

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

@@ -1191,7 +1191,7 @@ class CompilationUnitElementImpl extends UriReferencedElementImpl
   }
 
   @override
-  ClassElement? getClass(String className) {
+  ClassElementImpl? getClass(String className) {
     for (var class_ in classes) {
       if (class_.name == className) {
         return class_;
@@ -5900,11 +5900,11 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
 
   /// The non-nullable instance of this element, without alias.
   /// Should be used only when the element has no type parameters.
-  InterfaceType? _nonNullableInstance;
+  InterfaceTypeImpl? _nonNullableInstance;
 
   /// The nullable instance of this element, without alias.
   /// Should be used only when the element has no type parameters.
-  InterfaceType? _nullableInstance;
+  InterfaceTypeImpl? _nullableInstance;
 
   List<ConstructorElementImpl> _constructors = _Sentinel.constructorElement;
 
@@ -6077,7 +6077,7 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
   }
 
   @override
-  InterfaceType instantiate({
+  InterfaceTypeImpl instantiate({
     required List<DartType> typeArguments,
     required NullabilitySuffix nullabilitySuffix,
   }) {
@@ -7090,7 +7090,7 @@ class LibraryElementImpl extends ElementImpl
   }
 
   @override
-  ClassElement? getClass(String name) {
+  ClassElementImpl? getClass(String name) {
     for (var unitElement in units) {
       var element = unitElement.getClass(name);
       if (element != null) {