APIs are missing from some libraries/packages in stable

[DartBot]

This issue was originally filed by @butlermatt

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Some APIs are missing from the current stable documentation. For instance:
http://api.dartlang.org/docs/channels/stable/latest/observe.html

Currently shows the primary library documentation but there are no APIs listed like found in bleeding_edge
http://api.dartlang.org/docs/channels/be/latest/observe.html

[DartBot]

This comment was originally written by @butlermatt

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Note the same appear to also be missing from the DartDoc-next preview as well.
http://dartdoc-next.dartlang-api.appspot.com/apidocs/#observe/observe

[sethladd]

Added Area-DartDoc, Triaged labels.

[alan-knight]

The dart2js source mirrors have only recently acquired the ability to provide that information. Should work in dartdoc-next soon, we hope.

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Added Duplicate label.
Marked as being merged into #12759.