First partial step at removing interfaces in libraries.

lib/core/duration.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 /**
  * A [Duration] represents a time span. A duration can be negative.
  */
 interface Duration extends Comparable default DurationImplementation {
   /**
    * The duration is the sum of all individual parts. This means that individual
    * parts don't need to be less than the next-bigger unit. For example [hours]
    * is allowed to have a value greater than 23.
    *
    * All individual parts are allowed to be negative.
    * All arguments are by default 0.
    */
   const Duration([int days, int hours, int minutes, int seconds,
                   int milliseconds]);
 
- /**
+  /**
    * Returns this [Duration] in days. Incomplete days are discarded.
    */
-  final int inDays;
+  int get inDays;
 
   /**
    * Returns this [Duration] in hours. Incomplete hours are discarded.
    * The returned value can be greater than 23.
    */
-  final int inHours;
+  int get inHours;
 
   /**
    * Returns this [Duration] in minutes. Incomplete minutes are discarded.
    * The returned value can be greater than 59.
    */
-  final int inMinutes;
+  int get inMinutes;
 
   /**
    * Returns this [Duration] in seconds. Incomplete seconds are discarded.
    * The returned value can be greater than 59.
    */
-  final int inSeconds;
+  int get inSeconds;
 
   /**
    * Returns this [Duration] in milliseconds.
    */
-  final int inMilliseconds;
+  int get inMilliseconds;
 
   static const int MILLISECONDS_PER_SECOND = 1000;
   static const int SECONDS_PER_MINUTE = 60;
   static const int MINUTES_PER_HOUR = 60;
   static const int HOURS_PER_DAY = 24;
 
   static const int MILLISECONDS_PER_MINUTE =
       MILLISECONDS_PER_SECOND * SECONDS_PER_MINUTE;
   static const int MILLISECONDS_PER_HOUR =
       MILLISECONDS_PER_MINUTE * MINUTES_PER_HOUR;
   static const int MILLISECONDS_PER_DAY =
       MILLISECONDS_PER_HOUR * HOURS_PER_DAY;
 
   static const int SECONDS_PER_HOUR = SECONDS_PER_MINUTE * MINUTES_PER_HOUR;
   static const int SECONDS_PER_DAY = SECONDS_PER_HOUR * HOURS_PER_DAY;
 
   static const int MINUTES_PER_DAY = MINUTES_PER_HOUR * HOURS_PER_DAY;
 }