[analyzer] Change substitute methods to return TypeImpl.

The following methods are changed so that they return `TypeImpl`
rather than `DartType`:

- `FreshTypeParameters.substitute`
- `Substitution.substituteType`
- `_NullSubstitution.substituteType`

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.

Change-Id: Iaf4959cd7f6428d1dbe1b15089f7e2a30eff3495
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405080
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

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

@@ -11001,7 +11001,7 @@ class TypeAliasElementImpl extends _ExistingElementImpl
         ? NullabilitySuffix.question
         : nullabilitySuffix;
 
-    if (type is FunctionType) {
+    if (type is FunctionTypeImpl) {
       return FunctionTypeImpl(
         typeFormals: type.typeFormals,
         parameters: type.parameters,
@@ -11032,7 +11032,7 @@ class TypeAliasElementImpl extends _ExistingElementImpl
           typeArguments: typeArguments,
         ),
       );
-    } else if (type is TypeParameterType) {
+    } else if (type is TypeParameterTypeImpl) {
       return TypeParameterTypeImpl(
         element: type.element,
         nullabilitySuffix: resultNullability,
@@ -11042,7 +11042,7 @@ class TypeAliasElementImpl extends _ExistingElementImpl
         ),
       );
     } else {
-      return (type as TypeImpl).withNullability(resultNullability);
+      return type.withNullability(resultNullability);
     }
   }
 

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

@@ -180,7 +180,7 @@ class FreshTypeParameters {
     );
   }
 
-  DartType substitute(DartType type) => substitution.substituteType(type);
+  TypeImpl substitute(DartType type) => substitution.substituteType(type);
 }
 
 /// Substitution that is based on the [map].
@@ -205,9 +205,9 @@ abstract class Substitution {
     return types.map(mapInterfaceType);
   }
 
-  DartType substituteType(DartType type, {bool contravariant = false}) {
+  TypeImpl substituteType(DartType type, {bool contravariant = false}) {
     var visitor = _TopSubstitutor(this, contravariant);
-    return type.accept(visitor);
+    return type.accept(visitor) as TypeImpl;
   }
 
   /// Substitutes both variables from [first] and [second], favoring those from
@@ -390,7 +390,8 @@ class _NullSubstitution extends MapSubstitution {
   }
 
   @override
-  DartType substituteType(DartType type, {bool contravariant = false}) => type;
+  TypeImpl substituteType(DartType type, {bool contravariant = false}) =>
+      type as TypeImpl;
 
   @override
   String toString() => "Substitution.empty";