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

libraries/kotlin.test/junit/src/main/kotlin/JUnitSupport.kt

@@ -35,7 +35,9 @@ object JUnitAsserter : Asserter {
         Assert.assertNull(message ?: "actual value is not null", actual)
     }
 
-    override fun fail(message : String?) {
+    override fun fail(message : String?): Nothing {
         Assert.fail(message)
+        // should not get here
+        throw AssertionError(message)
     }
 }

libraries/kotlin.test/shared/src/main/kotlin.jvm/kotlin/test/TestAssertionsJVM.kt

@@ -22,11 +22,9 @@ fun <T : Throwable> assertFailsWith(exceptionClass: Class<T>, message: String?,
             return e as T
         }
         asserter.fail((message?.let { "$it. " } ?: "") + "Expected an exception of type $exceptionClass to be thrown, but was $e")
-        throw e
     }
     val msg = message?.let { "$it. " } ?: ""
     asserter.fail(msg + "Expected an exception of type $exceptionClass to be thrown, but was completed successfully.")
-    throw IllegalStateException(msg + "Should have failed.")
 }
 
 /** Asserts that a [block] fails with a specific exception of type [exceptionClass] being thrown. */

libraries/kotlin.test/shared/src/main/kotlin/kotlin/test/DefaultAsserter.kt

@@ -5,7 +5,7 @@ package kotlin.test
  */
 class DefaultAsserter() : Asserter {
 
-    override fun fail(message: String?) {
+    override fun fail(message: String?): Nothing {
         if (message == null)
             throw AssertionError()
         else

libraries/kotlin.test/shared/src/main/kotlin/kotlin/test/TestAssertions.kt

@@ -63,7 +63,7 @@ fun assertNull(actual: Any?, message: String? = null) {
 }
 
 /** Marks a test as having failed if this point in the execution path is reached, with an optional [message]. */
-fun fail(message: String? = null) {
+fun fail(message: String? = null): Nothing {
     asserter.fail(message)
 }
 
@@ -81,16 +81,13 @@ fun <@OnlyInputTypes T> expect(expected: T, message: String?, block: () -> T) {
 fun fails(block: () -> Unit): Throwable? = assertFails(block)
 
 /** Asserts that given function [block] fails by throwing an exception. */
-fun assertFails(block: () -> Unit): Throwable? {
-    var thrown: Throwable? = null
+fun assertFails(block: () -> Unit): Throwable {
     try {
         block()
     } catch (e: Throwable) {
-        thrown = e
+        return e
     }
-    if (thrown == null)
-        asserter.fail("Expected an exception to be thrown")
-    return thrown
+    asserter.fail("Expected an exception to be thrown")
 }
 
 /**
@@ -103,7 +100,7 @@ interface Asserter {
      *
      * @param message the message to report.
      */
-    fun fail(message: String?): Unit
+    fun fail(message: String?): Nothing
 
     /**
      * Asserts that the specified value is `true`.