First partial step at removing interfaces in libraries.

lib/core/duration.dart

Index: lib/core/duration.dart
===================================================================
--- lib/core/duration.dart	(revision 12179)
+++ lib/core/duration.dart	(working copy)
@@ -19,33 +19,33 @@
   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;