Improve List implementation.

dart/frog/leg/lib/mock.dart

Index: dart/frog/leg/lib/mock.dart
diff --git a/dart/frog/leg/lib/mock.dart b/dart/frog/leg/lib/mock.dart
index fe97f98a5517cf700cd307d98a4e5e7d4f7abd67..7d679dde9d8a0c6d661d447028dc7da2d34264af 100644
--- a/dart/frog/leg/lib/mock.dart
+++ b/dart/frog/leg/lib/mock.dart
@@ -20,11 +20,4 @@ class InternalError {
 // TODO(ahe): VM specfic exception?
 class StaticResolutionException implements Exception {}
 
-class List<E> implements Iterable<E> {
-  factory List([int length]) => Primitives.newList(length);
-  factory List.from(Iterable<E> other) {
-    throw 'List.from is not implemented';
-  }
-}
-
 void assert(condition) {}