Use `Function` as function type syntax.

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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 dart2js.cmdline;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io' show exit, File, FileMode, Platform, stdin, stderr;
 
@@ skipping 22 matching lines @@
  *
  * The actual string is rewritten by a wrapper script when included in the sdk.
  */
 String BUILD_ID = null;
 
 /**
  * The data passed to the [HandleOption] callback is either a single
  * string argument, or the arguments iterator for multiple arguments
  * handlers.
  */
-typedef void HandleOption(data);
+typedef HandleOption = void Function(dynamic data);
 
 class OptionHandler {
   final String pattern;
   final HandleOption handle;
   final bool multipleArguments;
 
   OptionHandler(this.pattern, this.handle, {this.multipleArguments: false});
 }
 
 /**
@@ skipping 700 matching lines @@
 void main(List<String> arguments) {
   // Since the sdk/bin/dart2js script adds its own arguments in front of
   // user-supplied arguments we search for '--batch' at the end of the list.
   if (arguments.length > 0 && arguments.last == "--batch") {
     batchMain(arguments.sublist(0, arguments.length - 1));
     return;
   }
   internalMain(arguments);
 }
 
-typedef void ExitFunc(int exitCode);
-typedef Future<api.CompilationResult> CompileFunc(
+typedef ExitFunc = void Function(int exitCode);
+typedef CompileFunc = Future<api.CompilationResult> Function(
     CompilerOptions compilerOptions,
     api.CompilerInput compilerInput,
     api.CompilerDiagnostics compilerDiagnostics,
     api.CompilerOutput compilerOutput);
 
 ExitFunc exitFunc = exit;
 CompileFunc compileFunc = api.compile;
 
 Future<api.CompilationResult> internalMain(List<String> arguments) {
   Future onError(exception, trace) {
@@ skipping 255 matching lines @@
   @override
   void close() {
     // Do nothing.
   }
 
   @override
   void addError(errorEvent, [StackTrace stackTrace]) {
     // Ignore
   }
 }