Update library and compiler with @unchecked

Update library and compiler with @unchecked annotations when matching
against opaque types.

Author: odersky

compiler/src/dotty/tools/dotc/core/unpickleScala2/PickleBuffer.scala

@@ -241,10 +241,10 @@ object PickleBuffer {
     val termMap, typeMap = new Array[Long](64)
     for (idx <- 0 until ScalaFlagEnd)
       corr get (1L << idx) match {
-        case Some((termFlag: FlagSet, typeFlag: FlagSet)) =>
+        case Some((termFlag: FlagSet @unchecked, typeFlag: FlagSet @unchecked)) =>
           termMap(idx) |= termFlag.bits
           typeMap(idx) |= typeFlag.bits
-        case Some(commonFlag: FlagSet) =>
+        case Some(commonFlag: FlagSet @unchecked) =>
           termMap(idx) |= commonFlag.toTermFlags.bits
           typeMap(idx) |= commonFlag.toTypeFlags.bits
         case _ =>

library/src/scala/Tuple.scala

@@ -216,7 +216,7 @@ object Tuple {
   /** Convert an immutable array into a tuple of unknown arity and types */
   def fromIArray[T](xs: IArray[T]): Tuple = {
     val xs2: IArray[Object] = xs match {
-      case xs: IArray[Object] => xs
+      case xs: IArray[Object] @unchecked => xs
       case xs =>
         // TODO support IArray.map
         xs.asInstanceOf[Array[T]].map(_.asInstanceOf[Object]).asInstanceOf[IArray[Object]]