Server notification to indicate `pub list-dir` fallback.
Server notification to indicate `pub list-dir` fallback.

For diagnostic purposes.

See: https://github.com/dart-lang/sdk/issues/24900 for more context.

BUG=24900
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org/1438293002 .

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