Export SyntheticBeginToken and SyntheticToken from the analyzer.
Export SyntheticBeginToken and SyntheticToken from the analyzer.

These classes are needed by the angular_analyzer_plugin project, and
we don't want to make it depend directly on front_end, so we need to
re-export the classes from the analyzer.  See discussion at
https://github.com/dart-lang/angular_analyzer_plugin/pull/558#discussion_r183825214

Change-Id: Ide0264e3e04f783eb9f68079cc2de8a6acfb94b3
Reviewed-on: https://dart-review.googlesource.com/52425
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

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