This CL uses the recently added 'include' tag in libraries
specification to share common library definitions between targets
'dart2js' and 'dart2js_server' through the "private" target
'_dart2js_common'.
Change-Id: I12f85d0d71803c6adcb94e37385ba837f7cdc9ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/224953
Reviewed-by: Sigmund Cherem <sigmund@google.com>