Fix implicit cast warning

pkg/kernel/lib/binary/ast_to_binary.dart

 // Copyright (c) 2016, 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 kernel.ast_to_binary;
 
 import '../ast.dart';
 import '../import_table.dart';
 import 'tag.dart';
 import 'dart:convert';
 import 'dart:typed_data';
@@ skipping 1017 matching lines @@
   visitTypeParameter(TypeParameter node) {
     writeStringReference(node.name ?? '');
     writeNode(node.bound);
   }
 
   defaultNode(Node node) {
     throw 'Unsupported node: $node';
   }
 }
 
+typedef bool LibraryFilter(Library _);
+
 /// A [LibraryFilteringBinaryPrinter] can write a subset of libraries.
 ///
 /// This printer writes a Kernel binary but includes only libraries that match a
 /// predicate.
 class LibraryFilteringBinaryPrinter extends BinaryPrinter {
-  final Function predicate;
+  final LibraryFilter predicate;
 
   LibraryFilteringBinaryPrinter(
       Sink<List<int>> sink, bool predicate(Library library))
       : predicate = predicate,
         super(sink);
 
   void writeProgramFile(Program program) {
     program.computeCanonicalNames();
     writeMagicWord(Tag.ProgramFile);
     _stringIndexer.scanProgram(program);
@@ skipping 295 matching lines @@
   void flush() {
     _sink.add(_buffer.sublist(0, length));
     _buffer = new Uint8List(SIZE);
     length = 0;
   }
 
   void flushAndDestroy() {
     _sink.add(_buffer.sublist(0, length));
   }
 }