Make first and last getters.

lib/core/queue.dart

Index: lib/core/queue.dart
diff --git a/lib/core/queue.dart b/lib/core/queue.dart
index beff3d7b9942e9e632d43bd9cc999fc8c1fd488c..e217e76bedb4558af743a976bd536d52df4656d4 100644
--- a/lib/core/queue.dart
+++ b/lib/core/queue.dart
@@ -57,13 +57,13 @@ abstract class Queue<E> extends Collection<E> {
    * Returns the first element of the queue. Throws an
    * [EmptyQueueException] exception if this queue is empty.
    */
-  E first();
+  E get first;
 
   /**
    * Returns the last element of the queue. Throws an
    * [EmptyQueueException] exception if this queue is empty.
    */
-  E last();
+  E get last;
 
   /**
    * Removes all elements in the queue. The size of the queue becomes zero.