Enable experimental mode when experimental feature is imported

[nicolasstucki]

Make it easier to use experimental language features out of the box. We already have -experimental that enables experimental mode globally by adding the @experimental annotation to all top-level definitions. The complication was that this flag was necessary to use experimental language features. It was not always easy to set (for example in the REPL) and it is all or nothing. Users can also annotate explicitly all top-level definitions that use language features with @experimental.

Now, the import of an experimental language feature will automatically enable experimental mode. If the import is at the top-level, all definitions in the file are experimental.

We also remove the special case that makes all code generated with a nightly or snapshot compiler compile as if in experimental mode. This implies that now the behavior of nightlies is identical to the behavior of release versions. The -Yno-experimental becomes a no-op and will be removed in a later PR.

[test_scala2_library_tasty]

[odersky]

Otherwise LGTM

[odersky]

Instead of the TOPLEVEL_SUFFIX test we can do sym.name.isPackageObjectName. But why do it at all, since we already have an isPackageObject test. Is that not redundant?

[nicolasstucki]

Removed the redundant test

[odersky]

Consider combining the two definitions into:

pack.info.decls.toList.iterator.flatMap: sym =>
  if sym.isClass && (sym.isPackage or sym.isPackageObject) then 
     nonExperimentalTopLevelDefs(sym)
  else if isNonExperimentalTopLevelDefinition(sym) then sym :: Nil
  else Nil

[nicolasstucki]

Updated

[odersky]

LGTM!