Make List.remove return boolean.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 1ebc5b7cfd826627c4453fda8089edc8b8910544..6e038881bc3c84926de4b35d1afdb483ce7e88ec 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -43,7 +43,7 @@ class _ObjectArray<E> implements List<E> {
         "Cannot remove element of a non-extendable array");
   }
 
-  void remove(Object element) {
+  bool remove(Object element) {
     throw new UnsupportedError(
         "Cannot remove element of a non-extendable array");
   }
@@ -310,7 +310,7 @@ class _ImmutableArray<E> implements List<E> {
         "Cannot modify an immutable array");
   }
 
-  void remove(Object element) {
+  bool remove(Object element) {
     throw new UnsupportedError(
         "Cannot modify an immutable array");
   }