Moved MutableMap.set from Maps.kt to MapsJVM.kt to avoid ambiguity when compile MapTest.kt by JS backend.

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

@@ -8,8 +8,9 @@ import java.lang.*;
 native
 public val noImpl : Nothing = throw Exception()
 
+// Drop this after KT-2093 will be fixed and restore MutableMap.set in Maps.kt from MapsJVM.kt
 /** Provides [] access to maps */
-native public fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit = noImpl
+native public fun <K, V> MutableMap<K, V>.set(key: K, value: V): V? = noImpl
 
 library("println")
 public fun println() {}
@@ -22,4 +23,4 @@ native public fun parseInt(s: String, radix: Int = 10): Int = js.noImpl
 library
 public fun safeParseInt(s : String) : Int? = js.noImpl
 library
-public fun safeParseDouble(s : String) : Double? = js.noImpl
+public fun safeParseDouble(s : String) : Double? = js.noImpl