Use `Function` as function type syntax.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 352fc9f9802582e3aad678cf5929c3b20851e265..d8f10d1a6a5d18e3eb5dc9f8c3fcf78797270cc6 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -40,7 +40,7 @@ String BUILD_ID = null;
  * 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;
@@ -761,8 +761,8 @@ void main(List<String> arguments) {
   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,