Changed interfaces to abstract classes where possible in {lib,samples,pkg}

lib/isolate/base.dart

Index: lib/isolate/base.dart
===================================================================
--- lib/isolate/base.dart	(revision 12252)
+++ lib/isolate/base.dart	(working copy)
@@ -50,7 +50,7 @@
  *
  * [SendPort]s can be transmitted to other isolates.
  */
-interface SendPort extends Hashable {
+abstract class SendPort implements Hashable {
 
   /**
    * Sends an asynchronous [message] to this send port. The message is copied to
@@ -137,7 +137,7 @@
 }
 
 // TODO(kasperl): Make this hashable and document it.
-interface SendPortSync {
+abstract class SendPortSync {
 
   callSync(var message);