Improve the error messages for IndexOutOfRangeException.

runtime/lib/arrays.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(ngeoffray): Rename to Lists.
 class Arrays {
   static void copy(List src, int srcStart,
                    List dst, int dstStart, int count) {
     if (srcStart === null) srcStart = 0;
     if (dstStart === null) dstStart = 0;
@@ skipping 63 matching lines @@
       }
     }
     return -1;
   }
 
   static void rangeCheck(List a, int start, int length) {
     if (length < 0) {
       throw new IllegalArgumentException("negative length $length");
     }
     if (start < 0 || start >= a.length) {
-      throw new IndexOutOfRangeException(start);
+      String message = "$start must be in the range [0..${a.length})"; 
+      throw new IndexOutOfRangeException(message);
     }
     if (start + length > a.length) {
-      throw new IndexOutOfRangeException(start + length);
+      String message = "$start + $length must be in the range [0..${a.length})";
+      throw new IndexOutOfRangeException(message);
     }
   }
 }