Implement type variables on old style function-typed formal parameters.

pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.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.
 
 library fasta.kernel_function_type_builder;
 
 import 'package:kernel/ast.dart'
     show
         DartType,
         DynamicType,
         FunctionType,
         NamedType,
         Supertype,
         TypeParameter;
 
 import 'kernel_builder.dart'
     show
         FormalParameterBuilder,
         FunctionTypeBuilder,
         KernelFormalParameterBuilder,
         KernelTypeBuilder,
         KernelTypeVariableBuilder,
         LibraryBuilder,
         TypeVariableBuilder;
 
 class KernelFunctionTypeBuilder extends FunctionTypeBuilder
     implements KernelTypeBuilder {
+  final int charOffset;
+
   KernelFunctionTypeBuilder(
-      int charOffset,
+      this.charOffset,
       Uri fileUri,
       KernelTypeBuilder returnType,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals)
       : super(charOffset, fileUri, returnType, typeVariables, formals);
 
   DartType build(LibraryBuilder library) {
     DartType builtReturnType =
         returnType?.build(library) ?? const DynamicType();
     List<DartType> positionalParameters = <DartType>[];
@@ skipping 27 matching lines @@
         requiredParameterCount: requiredParameterCount);
   }
 
   Supertype buildSupertype(LibraryBuilder library) {
     library.addCompileTimeError(
         charOffset, "Can't use a function type as supertype.",
         fileUri: fileUri);
     return null;
   }
 }