JS backend: make MutableMap.set as function intrinsic.

Fixed test WebDemoExamples2Test#builder.

js/js.libraries/src/core/javautil.kt

@@ -2,6 +2,10 @@ package java.util
 
 import java.lang.*
 
+/** Provides [] access to maps */
+native
+public fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit = noImpl
+
 library("collectionsMax")
 public fun max<T>(col : Collection<T>, comp : Comparator<T>) : T = js.noImpl