Fix warning in element_handle.dart.
Fix warning in element_handle.dart.

Commit 4bd6b4b0485023785ebe89fae59df3d33495ca3b added the abstract
method appendToWithoutDelimiters() to ParameterElement, which is
implemented in ParameterElementMixin.  However, one derived class was
not updated to make use of the new mixin: ParameterElementHandle.

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//1308713004 .

dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk