kotlin.test: 'fail' returns Nothing, 'assertFails' returns Throwable (not nullable).

Js tests now fail at the first failed assertion.
#KT-10289 Fixed
#KT-10369 Fixed

js/js.translator/testData/jsTester/jsTester.js

@@ -21,11 +21,15 @@ var JsTests = (function () {
 
     var assert = function (isTrue, message) {
         if (!isTrue) {
-            reporter.reportError(message);
-            throw failedTest;
+            fail(message);
         }
     };
 
+    var fail = function (message) {
+        reporter.reportError(message);
+        throw failedTest;
+    };
+
     var init = function () {
         init = function() {};
         Kotlin.modules["JS_TESTS"].kotlin.test.init();
@@ -49,6 +53,7 @@ var JsTests = (function () {
     };
     return {
         test: test,
-        assert: assert
+        assert: assert,
+        fail: fail
     }
 })();

js/js.translator/testData/jsTester/jsTester.kt

@@ -7,10 +7,11 @@ fun init() {
 }
 
 public class JsTestsAsserter() : Asserter {
-    public override fun fail(message: String?) {
-        assert(false, message)
-    }
+    public override fun fail(message: String?): Nothing = failWithMessage(message)
 }
 
 @native("JsTests.assert")
 public fun assert(value: Boolean, message: String?): Unit = noImpl
+
+@native("JsTests.fail")
+private fun failWithMessage(message: String?): Nothing = noImpl