IllegalArgumentException expects a string.

runtime/lib/array.dart

Index: runtime/lib/array.dart
===================================================================
--- runtime/lib/array.dart	(revision 400)
+++ runtime/lib/array.dart	(working copy)
@@ -93,7 +93,9 @@
       native "ObjectArray_copyFromObjectArray";
 
   void setRange(int start, int length, List<T> from, [int startFrom = 0]) {
-    if (length < 0) throw new IllegalArgumentException(length);
+    if (length < 0) {
+      throw new IllegalArgumentException("negative length $length");
+    }
     copyFrom(from, start, startFrom, count);
   }