Emit empty .tasty files by default

Before this commit, we only created .tasty files under -YemitTasty, we
haven't decided yet if this should be the default, but even if we don't
end up doing that, it is still useful to emit empty .tasty files to
signal that the corresponding .class file has a tasty section, this is
much simpler to check than parsing classfiles to look for a tasty section.

Also, fix -YemitTasty tof work with AbstractFile who are not backed by
java.io.File, previously this lead to NullPointerException.

Author: smarter

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -25,7 +25,7 @@ import Symbols._
 import Denotations._
 import Phases._
 import java.lang.AssertionError
-import java.io.{FileOutputStream, File => JFile}
+import java.io.{DataOutputStream, File => JFile}
 
 import scala.tools.asm
 import scala.tools.asm.tree._
@@ -205,12 +205,16 @@ class GenBCodePipeline(val entryPoints: List[Symbol], val int: DottyBackendInter
             val dataAttr = new CustomAttr(nme.TASTYATTR.mangledString, binary)
             val store = if (mirrorC ne null) mirrorC else plainC
             store.visitAttribute(dataAttr)
+            val outTastyFile = getFileForClassfile(outF, store.name, ".tasty")
             if (ctx.settings.emitTasty.value) {
-              val outTastyFile = getFileForClassfile(outF, store.name, ".tasty").file
-              val fos = new FileOutputStream(outTastyFile, false)
-              fos.write(binary)
-              fos.close()
-
+              val outstream = new DataOutputStream(outTastyFile.bufferedOutput)
+
+              try outstream.write(binary)
+              finally outstream.close()
+            } else if (!outTastyFile.isVirtual) {
+              // Create an empty file to signal that a tasty section exist in the corresponding .class
+              // This is much cheaper and simpler to check than doing classfile parsing
+              outTastyFile.create()
             }
           }