Fix assertion in erasure to correctly print name of the phase.

Author: DarkDimius

src/dotty/tools/dotc/transform/Erasure.scala

@@ -109,7 +109,7 @@ class Erasure extends Phase with DenotTransformer { thisTransformer =>
   }
 
   def assertErased(tp: Type, tree: tpd.Tree = tpd.EmptyTree)(implicit ctx: Context): Unit =
-    assert(isErasedType(tp), i"The type $tp - ${tp.toString} of class ${tp.getClass} of tree $tree : ${tree.tpe} / ${tree.getClass} is illegal after erasure, phase = ${ctx.phase}")
+    assert(isErasedType(tp), i"The type $tp - ${tp.toString} of class ${tp.getClass} of tree $tree : ${tree.tpe} / ${tree.getClass} is illegal after erasure, phase = ${ctx.phase.prev}")
 }
 
 object Erasure extends TypeTestsCasts{