Fix 16099: Invocation's positionalArguments contains an immutable array.

runtime/lib/invocation_mirror_patch.dart

Index: runtime/lib/invocation_mirror_patch.dart
===================================================================
--- runtime/lib/invocation_mirror_patch.dart	(revision 31835)
+++ runtime/lib/invocation_mirror_patch.dart	(working copy)
@@ -67,7 +67,8 @@
         return _positionalArguments = const [];
       }
       // Exclude receiver.
-      _positionalArguments = _arguments.sublist(1, numPositionalArguments);
+      _positionalArguments =
+          new _ImmutableList._from(_arguments, 1, numPositionalArguments - 1);
     }
     return _positionalArguments;
   }