Workaround for type propagation error

We had
```
def classSymbols(implicit ctx: Context): List[ClassSymbol] =
  parentSymbols(_.isClass).asInstanceOf
```
Under significant indentation, the body of the method is conceptually
a block. That causes the type parameter of `asInstanceOf` to be inferred
to `Nothing` instead of `List[ClassSymbol]`. The same effect occurs
if we enclose the body in braces (even under -noindent).

I will try to fix the type propagation fail in a separate commit.
This here is a workaround.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -458,7 +458,7 @@ object Types {
     /** The least (wrt <:<) set of class symbols of which this type is a subtype
      */
     final def classSymbols(implicit ctx: Context): List[ClassSymbol] =
-      parentSymbols(_.isClass).asInstanceOf
+      parentSymbols(_.isClass).asInstanceOf[List[ClassSymbol]]
 
     /** The term symbol associated with the type */
     @tailrec final def termSymbol(implicit ctx: Context): Symbol = this match {