JS backend: fixed Map set/get -- should work with jet Map/MutableMap.

Added to PatternBuilder the ability to check that method has been overridden.

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

@@ -5,9 +5,11 @@ import js.native
 import java.util.*;
 import java.lang.*;
 
-
 native
-public val noImpl : Nothing = throw Exception();
+public val noImpl : Nothing = throw Exception()
+
+/** Provides [] access to maps */
+native public fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit = noImpl
 
 library("println")
 public fun println() {}

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

@@ -1,11 +1,5 @@
 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