Use `Function` as function type syntax.

pkg/compiler/lib/src/dart_types.dart

 // Copyright (c) 2012, 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 dart_types;
 
 import 'dart:math' show min;
 
 import 'common/resolution.dart' show Resolution;
 import 'common.dart';
@@ skipping 1262 matching lines @@
   bool invalidCallableType(DartType callType, DartType s) {
     return !isSubtype(callType, s);
   }
 }
 
 /**
  * Callback used to check whether the [typeArgument] of [type] is a valid
  * substitute for the bound of [typeVariable]. [bound] holds the bound against
  * which [typeArgument] should be checked.
  */
-typedef void CheckTypeVariableBound(GenericType type, DartType typeArgument,
+typedef CheckTypeVariableBound = void Function(GenericType type, DartType typeArgument,
     TypeVariableType typeVariable, DartType bound);
 
 /// Basic interface for the Dart type system.
 abstract class DartTypes {
   /// The types defined in 'dart:core'.
   CoreTypes get coreTypes;
 
   /// Returns `true` if [t] is a subtype of [s].
   bool isSubtype(DartType t, DartType s);
 
@@ skipping 725 matching lines @@
           sb.write(', ');
         }
         namedParameterTypes[index].accept(this, namedParameters[index]);
         needsComma = true;
       }
       sb.write('}');
     }
     sb.write(')');
   }
 }