Open patch file for writing later

Open patch file for writing later only once all patches are applied,
so that any assertion errors during patch generation cannot result
in an empty output file.

Author: odersky

compiler/src/dotty/tools/dotc/rewrites/Rewrites.scala

@@ -51,9 +51,9 @@ object Rewrites {
     }
 
     def writeBack(): Unit = {
-      val out = source.file.output
       val chars = apply(source.underlying.content)
       val bytes = new String(chars).getBytes
+      val out = source.file.output
       out.write(bytes)
       out.close()
     }