JS: fix toString in case of kotlin.Throwable

js/js.libraries/src/core/builtins.kt

@@ -61,7 +61,13 @@ internal fun captureStack(baseClass: JsClass<in Throwable>, instance: Throwable)
 @JsName("newThrowable")
 internal fun newThrowable(message: String?, cause: Throwable?): Throwable {
     val throwable = js("new Error()")
-    throwable.message = if (jsTypeOf(message) == "undefined" && cause != null) cause.toString() else message
+    throwable.message = if (jsTypeOf(message) == "undefined") {
+        if (cause != null) cause.toString() else null
+    }
+    else {
+        message
+    }
     throwable.cause = cause
+    throwable.name = "Throwable"
     return throwable
 }

js/js.translator/testData/box/expression/try/exceptionToString.kt

@@ -13,6 +13,8 @@ fun check(e: Throwable, expectedString: String) {
 }
 
 fun box(): String {
+    check(Throwable(), "Throwable: null")
+    check(Throwable("ccc"), "Throwable: ccc")
     check(Exception(), "Exception: null")
     check(Exception("bbb"), "Exception: bbb")
     check(MyException(), "MyException: null")