[cfe] Ensure inferred parameter types on intermediate mixin application constructors

Closes #46159

Change-Id: Ice569565db68832c47cffb745faaca39defd9be2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206641
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

@@ -716,7 +716,9 @@ class KernelTarget extends TargetImplementation {
       Constructor? referenceFrom) {
     VariableDeclaration copyFormal(VariableDeclaration formal) {
       VariableDeclaration copy = new VariableDeclaration(formal.name,
-          isFinal: formal.isFinal, isConst: formal.isConst);
+          isFinal: formal.isFinal,
+          isConst: formal.isConst,
+          type: const UnknownType());
       if (formal.type is! UnknownType) {
         copy.type = substitute(formal.type, substitutionMap);
       } else {
@@ -1424,7 +1426,7 @@ class DelayedParameterType {
 
   void updateType() {
     // ignore: unnecessary_null_comparison
-    assert(source.type != null, "No type computed for $source.");
+    assert(source.type is! UnknownType, "No type computed for $source.");
     target.type = substitute(source.type, substitutionMap);
   }
 }

pkg/front_end/testcases/incremental/type_change_on_recompile.yaml.world.1.expect

@@ -43,7 +43,7 @@ library from "org-dartlang-test:///main.dart" as main {
       ;
   }
   abstract class _A&B&C&D extends main::_A&B&C implements main::D /*isAnonymousMixin,isEliminatedMixin*/  {
-    synthetic constructor named(dynamic _field) → main::_A&B&C&D*
+    synthetic constructor named(lib::E* _field) → main::_A&B&C&D*
       : super main::_A&B&C::named(_field)
       ;
   }