Make List.remove return boolean.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index 1ebe8c2878425345dcfb55b484520fda0ac78d78..95594b60c81805cc6ed27cfdb59a5ab4f1ac1e47 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -43,13 +43,14 @@ class _GrowableObjectArray<T> implements List<T> {
     return result;
   }
 
-  void remove(Object element) {
+  bool remove(Object element) {
     for (int i = 0; i < this.length; i++) {
       if (this[i] == element) {
         removeAt(i);
-        return;
+        return true;
       }
     }
+    return false;
   }
 
   void insertAll(int index, Iterable<T> iterable) {