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.libraries/src/core/javalang.kt

@@ -27,6 +27,9 @@ public class NumberFormatException(message: String? = null) : RuntimeException(m
 @library
 public class NullPointerException(message: String? = null) : RuntimeException(message) {}
 
+@library
+public class AssertionError(message: String? = null) : Error(message) {}
+
 @library
 public interface Runnable {
     public open fun run() : Unit;

js/js.libraries/src/stdlib/testCode.kt

@@ -18,14 +18,23 @@ public var asserter: Asserter = QUnitAsserter()
 public class QUnitAsserter(): Asserter {
 
     public override fun assertTrue(lazyMessage: () -> String?, actual: Boolean) {
-        QUnit.ok(actual, lazyMessage())
+        assertTrue(actual, lazyMessage())
     }
 
     public override fun assertTrue(message: String?, actual: Boolean) {
         QUnit.ok(actual, message)
+        if (!actual) failWithMessage(message)
     }
 
-    public override fun fail(message: String?) {
+    public override fun fail(message: String?): Nothing {
         QUnit.ok(false, message)
+        failWithMessage(message)
+    }
+
+    private fun failWithMessage(message: String?): Nothing {
+        if (message == null)
+            throw AssertionError()
+        else
+            throw AssertionError(message)
     }
 }