Fix #651 Generalize criterion for isOpAssign

Methods like + can have multiple parameters. In that case
+= also takes multiple parameters.

Author: odersky

src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -33,7 +33,7 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
   }
 
   def isOpAssign(tree: Tree) = unsplice(tree) match {
-    case Apply(fn, _ :: Nil) =>
+    case Apply(fn, _ :: _) =>
       unsplice(fn) match {
         case Select(_, name) if name.isOpAssignmentName => true
         case _ => false

tests/pending/run/colltest1.check renamed to tests/run/colltest1.check

@@ -139,7 +139,8 @@ object Test extends dotty.runtime.LegacyApp {
 
   def setTest(empty: => Set[String]): Unit = {
     var s = empty + "A" + "B" + "C"
-    s += ("D", "E", "F")
+    s += "D"
+    s = s + ("D", "E", "F")
     s ++= List("G", "H", "I")
     s ++= ('J' to 'Z') map (_.toString)
     assert(s forall (s contains))