pkg/compiler: fix imports

pkg/compiler/lib/src/null_compiler_output.dart

 // Copyright (c) 2015, 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.
 
 /// Null pattern implementation of the [CompilerOutput] interface.
 
 library compiler.null_api;
 
+import 'dart:async';
+
 import '../compiler_new.dart';
-import 'dart:async';
 
 /// Null pattern implementation of the [CompilerOutput] interface.
 class NullCompilerOutput implements CompilerOutput {
   const NullCompilerOutput();
 
   @override
   EventSink<String> createEventSink(String name, String extension) {
     return NullSink.outputProvider(name, extension);
   }
 }
@@ skipping 10 matching lines @@
 
   void close() {}
 
   toString() => name;
 
   /// Convenience method for getting an [api.CompilerOutputProvider].
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }