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

pkg/unittest/interfaces.dart

Index: pkg/unittest/interfaces.dart
===================================================================
--- pkg/unittest/interfaces.dart	(revision 12252)
+++ pkg/unittest/interfaces.dart	(working copy)
@@ -23,7 +23,7 @@
  * other implementations, but could be useful to replace in
  * some cases - e.g. language conversion.
  */
-interface Description {
+abstract class Description {
   /** Change the value of the description. */
   Description replace(String text);
 
@@ -49,7 +49,7 @@
  * required. The other two methods ([matches] and [describe])
  * must always be provided as they are highly matcher-specific.
  */
-interface Matcher {
+abstract class Matcher {
   /**
    * This does the matching of the actual vs expected values.
    * [item] is the actual value. [matchState] can be supplied
@@ -81,7 +81,7 @@
  * this can be replaced by some other implementation of
  * IFailureHandler by calling configureExpectHandler.
  */
-interface FailureHandler {
+abstract class FailureHandler {
   /** This handles failures given a textual decription */
   void fail(String reason);