Using 'interface' and 'abstract' for methods should produce error

pkg/dartdoc/lib/mirrors.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('mirrors');
 
 #import('dart:io');
 #import('dart:uri');
 
 // TODO(rnystrom): Use "package:" URL (#4968).
 #import('src/mirrors/dart2js_mirror.dart');
 
 /**
  * [Compilation] encapsulates the compilation of a program.
  */
-class Compilation {
+abstract class Compilation {
   /**
    * Creates a new compilation which has [script] as its entry point.
    */
   factory Compilation(Path script,
                       Path libraryRoot,
                       [Path packageRoot,
                        List<String> opts = const <String>[]]) {
     return new Dart2JsCompilation(script, libraryRoot, packageRoot, opts);
   }
 
@@ skipping 11 matching lines @@
   }
 
   /**
    * Returns the mirror system for this compilation.
    */
   final MirrorSystem mirrors;
 
   /**
    * Returns a future for the compiled JavaScript code.
    */
-  abstract Future<String> compileToJavaScript();
+  Future<String> compileToJavaScript();
 }
 
 /**
  * The main interface for the whole mirror system.
  */
 abstract class MirrorSystem {
   /**
    * Returns an unmodifiable map of all libraries in this mirror system.
    */
   Map<String, LibraryMirror> get libraries;
@@ skipping 441 matching lines @@
   /**
    * Returns the URI where the source originated.
    */
   Uri get uri;
 
   /**
    * Returns the text of this source.
    */
   String get text;
 }