Add groupingBy and eachCount sample.

libraries/stdlib/samples/test/samples/collections/collections.kt

@@ -59,6 +59,18 @@ class Collections {
             // same content as in namesByTeam map, but the map is mutable
             assertTrue(mutableNamesByTeam == namesByTeam)
         }
+
+        @Sample
+        fun groupingByEachCount() {
+            val words = "one two three four five six seven eight nine ten".split(' ')
+            val frequenciesByFirstChar = words.groupingBy { it.first() }.eachCount()
+            println("Counting first letters:")
+            assertPrints(frequenciesByFirstChar, "{o=1, t=3, f=2, s=2, e=1, n=1}")
+
+            val moreWords = "eleven twelve".split(' ')
+            val moreFrequencies = moreWords.groupingBy { it.first() }.eachCountTo(frequenciesByFirstChar.toMutableMap())
+            assertPrints(moreFrequencies, "{o=1, t=4, f=2, s=2, e=2, n=1}")
+        }
     }
 
 }