Add List.first.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 1fb17fb8ecf4d9927e21e4fbe38cf604fa7fbb67..5bef0d0db28ec84e79e2c396c37e0822799c5a66 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -138,6 +138,10 @@ class _ObjectArray<E> implements List<E> {
         "Cannot remove in a non-extendable array");
   }
 
+  E get first {
+    return this[0];
+  }
+
   E get last {
     return this[length - 1];
   }
@@ -285,6 +289,10 @@ class _ImmutableArray<E> implements List<E> {
         "Cannot remove in a non-extendable array");
   }
 
+  E get first {
+    return this[0];
+  }
+
   E get last {
     return this[length - 1];
   }