IR JS: support findAssociatedObject feature (KT-37418 fixed)

libraries/stdlib/js-ir/src/kotlin/reflection_js-ir.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2020 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.
+ */
+
+import kotlin.reflect.*
+import kotlin.reflect.js.internal.*
+
+@PublishedApi
+internal fun <T : Annotation> KClass<*>.findAssociatedObject(annotationClass: KClass<T>): Any? {
+    return if (this is KClassImpl<*> && annotationClass is KClassImpl<T>) {
+        val key = annotationClass.jClass.asDynamic().`$metadata$`.associatedObjectKey?.unsafeCast<Int>() ?: return null
+        val map = this.jClass.asDynamic().`$metadata$`.associatedObjects ?: return null
+        val factory = map[key] ?: return null
+        return factory()
+    } else {
+        null
+    }
+}