Adds static errors in web backends requiring external members to be
annotated with @JS(). Members can be annotated directly, or on the
enclosing class or library.
Also removes duplicate checks in dart2js for non-native external
members that are now covered by the checks on external @JS members.
Change-Id: I2b17456f8f546f80f1ea6a817360c7f4c40f9640
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/180442
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Riley Porter <rileyporter@google.com>
Riley Porter