IR: copy each file before lowering.

Avoid inter-file dependencies while lowering.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsMapping.kt

@@ -47,5 +47,11 @@ class JsMapping(private val irFactory: IrFactory) : DefaultMapping() {
                 map[key] = value
             }
         }
+
+        override val keys: Set<K>
+            get() = map.keys
+
+        override val values: Collection<V>
+            get() = map.values
     }
 }