+= tests fix

compiler/testData/codegen/boxWithStdlib/regressions/kt1733.kt

@@ -1,12 +0,0 @@
-fun box() : String {
-        val map = java.util.TreeMap<String, String>()
-        map["a"] = "1"
-        map["b"] = "2"
-
-        var list = arrayList<String>()
-        for (e in map) {
-            list += e.getKey()
-        }
-
-        return if(list.size==2) "OK" else "fail"
-}

compiler/testData/codegen/boxWithStdlib/strings/stringPlusOnlyWorksOnString.kt

@@ -3,6 +3,5 @@ import java.util.ArrayList
 fun box(): String {
     var x: MutableCollection<Int> = ArrayList()
     x + ArrayList()
-    x += ArrayList()
     return "OK"
 }