[JS IR BE] Simplify throwable lowering.

- Make kotlin.Throwable class extenral
- Add runtime function 'extendThrowable' to use instead of delegating
  constructors to Throwable

libraries/stdlib/js-ir/build.gradle

@@ -28,7 +28,6 @@ task prepareNativeBuiltinsSources(type: Sync) {
         include "Nothing.kt"
         include "Number.kt"
         include "String.kt"
-        include "Throwable.kt"
     }
 
     into nativeBuiltinsSrcDir

libraries/stdlib/js-ir/builtins/Throwable.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin
+
+/**
+ * The base class for all errors and exceptions. Only instances of this class can be thrown or caught.
+ *
+ * @param message the detail message string.
+ * @param cause the cause of this throwable.
+ */
+@JsName("Error")
+public external open class Throwable {
+    open val message: String?
+    open val cause: Throwable?
+
+    constructor(message: String?, cause: Throwable?)
+    constructor(message: String?)
+    constructor(cause: Throwable?)
+    constructor()
+}

libraries/stdlib/js-ir/runtime/coreRuntime.kt

@@ -77,7 +77,6 @@ fun getStringHashCode(str: String): Int {
 
 fun identityHashCode(obj: Any?): Int = getObjectHashCode(obj)
 
-@JsName("captureStack")
 internal fun captureStack(instance: Throwable) {
     if (js("Error").captureStackTrace != null) {
         js("Error").captureStackTrace(instance, instance::class.js)
@@ -86,17 +85,19 @@ internal fun captureStack(instance: Throwable) {
     }
 }
 
-@JsName("newThrowable")
 internal fun newThrowable(message: String?, cause: Throwable?): Throwable {
     val throwable = js("new Error()")
-    throwable.message = if (message == null) {
-        if (cause != null) (cause.asDynamic().toString)() else null
-    } else {
-        message
-    }
+    throwable.message = message ?: cause?.toString()
     throwable.cause = cause
     throwable.name = "Throwable"
-    return throwable
+    return throwable.unsafeCast<Throwable>()
+}
+
+internal fun extendThrowable(this_: dynamic, message: String?, cause: Throwable?) {
+    js("Error").call(this_)
+    this_.message = message ?: cause?.toString()
+    this_.cause = cause
+    captureStack(this_)
 }
 
 internal fun <T, R> boxIntrinsic(x: T): R = error("Should be lowered")