Write-back uses unbuffered writer

Encoder has 8Kb buffer.

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

@@ -56,13 +56,11 @@ object Rewrites {
       ds
     }
 
-    def writeBack(): Unit = {
+    def writeBack(): Unit =
       val chars = apply(source.underlying.content)
-      val bytes = new String(chars).getBytes(UTF_8)
-      val out = source.file.output
-      out.write(bytes)
-      out.close()
-    }
+      val osw = OutputStreamWriter(source.file.output, UTF_8)
+      try osw.write(chars, 0, chars.length)
+      finally osw.close()
   }
 
   /** If -rewrite is set, record a patch that replaces the range