Provide the way to specify implicit default for a readonly or mutable map.

Refactor: rename default key and value providers.

libraries/stdlib/test/collections/MapTest.kt

@@ -3,6 +3,7 @@ package test.collections
 import kotlin.test.assertEquals
 import kotlin.test.assertFalse
 import kotlin.test.assertTrue
+import kotlin.test.fails
 import org.junit.Test as test
 
 class MapTest {
@@ -21,6 +22,27 @@ class MapTest {
         assertEquals(null, c)
     }
 
+    test fun getOrImplicitDefault() {
+        val data: MutableMap<String, Int> = hashMapOf("bar" to 1)
+        assertTrue(fails { data.getOrImplicitDefault("foo") } is KeyMissingException)
+        assertEquals(1, data.getOrImplicitDefault("bar"))
+
+        val mutableWithDefault = data.withDefault { 42 }
+        assertEquals(42, mutableWithDefault.getOrImplicitDefault("foo"))
+
+        // verify that it is wrapper
+        mutableWithDefault["bar"] = 2
+        assertEquals(2, data["bar"])
+        data["bar"] = 3
+        assertEquals(3, mutableWithDefault["bar"])
+
+        val readonlyWithDefault = (data as Map<String, Int>).withDefault { it.length() }
+        assertEquals(4, readonlyWithDefault.getOrImplicitDefault("loop"))
+
+        val withReplacedDefault = readonlyWithDefault.withDefault { 42 }
+        assertEquals(42, withReplacedDefault.getOrImplicitDefault("loop"))
+    }
+
     test fun getOrPut() {
         val data = hashMapOf<String, Int>()
         val a = data.getOrPut("foo") { 2 }