Don't ask defTree for Symbol's source after erasure

After erasure all trees get the source of the enclosing compilation
unit (via dropInlined). The defTrees don't change, however. Therefore
they are no longer correct for determing a symbol's source.

Author: odersky

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

@@ -624,7 +624,7 @@ object Symbols {
     type DontUseSymbolOnSymbol
 
     final def source(implicit ctx: Context): SourceFile =
-      if (!defTree.isEmpty) defTree.source
+      if (!defTree.isEmpty && !ctx.erasedTypes) defTree.source
       else this match {
         case cls: ClassSymbol => cls.sourceOfClass
         case _ =>