Changed interfaces to abstract classes in utils.

utils/archive/archive.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.
 
 /**
  * The primary library file for the archive library. This is the only file that
  * should be imported by clients.
  */
 #library("archive");
 
 #import("entry.dart", prefix: "entry");
 #import("reader.dart", prefix: "reader");
 
 // TODO(nweiz): Remove this when 3071 is fixed.
 /** An error raised by the archive library. */
-interface ArchiveException {
-  /** A description of the error that occurred. */   
+abstract class ArchiveException {
+  /** A description of the error that occurred. */
   final String message;
 
   /** The error code for the error, or null. */
   final int errno;
 }
 
 // TODO(nweiz): Remove this when 3071 is fixed.
 /** See [reader.ArchiveReader]. */
 class ArchiveReader extends reader.ArchiveReader {}
 
 // TODO(nweiz): Remove this when 3071 is fixed.
 /** See [entry.ArchiveEntry]. */
 class ArchiveEntry extends entry.ArchiveEntry {
   ArchiveEntry.internal(List properties, int archiveId)
     : super.internal(properties, archiveId);
 
   /** Create a new [ArchiveEntry] with default values for all of its fields. */
   static Future<ArchiveEntry> create() => entry.ArchiveEntry.create();
 }