made ArrayType constructor phase aware

Avoid to create ArrayTypes after erase.

Note that the *extractor* does not recognize JavaArrayTypes as
ArrayTypes. Doing so would create an infinite loop in sigName.
Generally, we do not want to ppaper over the difference when analysing
types.

Author: odersky

src/dotty/tools/dotc/core/Definitions.scala

@@ -394,7 +394,8 @@ class Definitions {
 
   object ArrayType {
     def apply(elem: Type)(implicit ctx: Context) =
-      ArrayClass.typeRef.appliedTo(elem :: Nil)
+      if (ctx.erasedTypes) JavaArrayType(elem)
+      else ArrayClass.typeRef.appliedTo(elem :: Nil)
     def unapply(tp: Type)(implicit ctx: Context) = tp.dealias match {
       case at: RefinedType if (at isRef ArrayClass) && at.argInfos.length == 1 => Some(at.argInfos.head)
       case _ => None