Improve the error messages for IndexOutOfRangeException.

runtime/lib/arrays.dart

Index: runtime/lib/arrays.dart
diff --git a/runtime/lib/arrays.dart b/runtime/lib/arrays.dart
index 012a9e82a1b72962884a09e63000d0b6ca059130..6cf0402accd0fe37c1a47a4b9a453cde9031db1e 100644
--- a/runtime/lib/arrays.dart
+++ b/runtime/lib/arrays.dart
@@ -81,10 +81,12 @@ class Arrays {
       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);
     }
   }
 }