JS backend: make MutableMap.set as function intrinsic.

Fixed test WebDemoExamples2Test#builder.

js/js.tests/test/org/jetbrains/k2js/test/semantics/WebDemoExamples2Test.java

@@ -33,9 +33,7 @@ public final class WebDemoExamples2Test extends SingleFileTranslationTest {
         performTestWithMain("life", "", "2");
     }
 
-    //TODO: fails because it need code from stdlib
-    @SuppressWarnings("UnusedDeclaration")
-    public void _testBuilder() throws Exception {
+    public void testBuilder() throws Exception {
         performTestWithMain("builder", "");
         performTestWithMain("builder", "1", "over9000");
     }