Adapt unpickling to the case where a parent is a block

It seems FromTasty printing has to do the same thing. For the moment
the test default-super.scala fails in the printing phase, and is
therefpre blacklisted.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -855,7 +855,7 @@ class TreeUnpickler(reader: TastyReader,
       val vparams = readIndexedParams[ValDef](PARAM)
       val parents = collectWhile(nextByte != SELFDEF && nextByte != DEFDEF) {
         nextUnsharedTag match {
-          case APPLY | TYPEAPPLY => readTerm()(parentCtx)
+          case APPLY | TYPEAPPLY | BLOCK => readTerm()(parentCtx)
           case _ => readTpt()(parentCtx)
         }
       }

compiler/test/dotc/pos-from-tasty.blacklist

@@ -12,3 +12,6 @@ annot-bootstrap.scala
 
 # ScalaRunTime cannot be unpickled because it is already loaded
 repeatedArgs213.scala
+
+# Error printing parent constructors that are blocks
+default-super.scala