[JS IR BE] Support 'is Number' and 'is Comparable<>'

libraries/stdlib/js/src/kotlin/currentBeMisc.kt

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. 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.js
+
+// Mirrors signature from JS IR BE
+// Used for js.translator/testData/box/number/mulInt32.kt
+@library
+@JsName("imulEmulated")
+internal fun imul(x: Int, y: Int): Int = definedExternally