Make ordering of check deterministic

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/CycleChecker.scala

@@ -84,7 +84,7 @@ class CycleChecker(cache: Cache) {
   private val proxyCache = mutable.Map.empty[Symbol, List[Dependency]]
 
   val classesInCurrentRun = mutable.Set.empty[Symbol]
-  val objectsInCurrentRun = mutable.Set.empty[Symbol]
+  val objectsInCurrentRun = new mutable.ListBuffer[Symbol]
 
   /** Checking state */
   case class State(visited: mutable.Set[Dependency], path: Vector[Symbol], trace: Vector[Dependency]) {

tests/init/neg/t9312.scala

@@ -15,7 +15,7 @@ object DeadLockTest {
       def siblings = parent.children - this
     }
 
-    object Child1 extends Child     // error
+    object Child1 extends Child
     object Child2 extends Child
 
     final val children = Set(Child1, Child2)