kotlin-stldib-js: do not import experimental sourceset in IDEA

Temporary workaround to prevent multiple IDEA modules sharing the same content root error.

libraries/stdlib/js/build.gradle

@@ -50,6 +50,7 @@ sourceSets {
     }
 
     experimental {
+        if(!System.properties.'idea.active')
         kotlin {
             srcDir experimentalSrcDir
         }