feat(polyfills): use stdlib for js polyfills.

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

@@ -0,0 +1,15 @@
+/*
+ * 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.
+ */
+@file:JsQualifier("Math")
+package kotlin.js
+
+@JsNativeImplementation("""
+if (typeof Math == "object" && Math !== null & typeof Math.imul === "undefined") {
+  Math.imul = function imul(a, b) {
+    return ((a & 0xffff0000) * (b & 0xffff) + (a & 0xffff) * (b | 0)) | 0; 
+  }
+}
+""")
+internal external fun imul(a_local: Int, b_local: Int): Int