Create KernelVariableDeclaration for formal parameters.

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 846 matching lines @@
   final TypePromotionFact<VariableDeclaration> _fact;
 
   final TypePromotionScope _scope;
 
   KernelVariableGet(VariableDeclaration variable, this._fact, this._scope)
       : super(variable);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    bool mutatedInClosure;
-    DartType declaredType;
-    var variable = this.variable;
-    if (variable is KernelVariableDeclaration) {
-      mutatedInClosure = variable._mutatedInClosure;
-      declaredType = variable._declaredType;
-    } else {
-      // Hack to deal with the fact that BodyBuilder still creates raw
-      // VariableDeclaration objects sometimes.
-      // TODO(paulberry): get rid of this once the type parameter is
-      // KernelVariableDeclaration.
-      mutatedInClosure = true;
-      declaredType = variable.type;
-    }
+    var variable = this.variable as KernelVariableDeclaration;
+    bool mutatedInClosure = variable._mutatedInClosure;
+    DartType declaredType = variable._declaredType;
     return inferrer.inferVariableGet(typeContext, typeNeeded, mutatedInClosure,
         _fact, _scope, fileOffset, declaredType, (type) {
       promotedType = type;
     });
   }
 }