Add regression tests

Author: nicolasstucki

tests/neg-custom-args/no-experimental/experimental-package-imports.scala

@@ -0,0 +1,24 @@
+import annotation.experimental
+
+package foo {
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+
+  package bar {
+    def foo = 1
+  }
+}
+
+package foo2 {
+  // ok: all definitions are top-level @experimental
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+
+  package bar {
+    @experimental def foo = 1
+  }
+}