properly deprecating previously WIP concept exercises

Removing from config.json is not the right strategy.
Refer to [Deprecated Exercises](https://exercism.org/docs/building/tracks/deprecated-exercises).
Relates to #600

Author: glennj

config.json

@@ -36,6 +36,32 @@
     ]
   },
   "exercises": {
+    "concept": [
+      {
+        "slug": "futures",
+        "name": "futures",
+        "uuid": "7bcc84a0-9209-497b-b32a-e0784885abc7",
+        "concepts": [],
+        "prerequisites": [],
+        "status": "deprecated"
+      },
+      {
+        "slug": "strings",
+        "name": "strings",
+        "uuid": "b6a1361a-7ae1-4265-b292-9570cb51bda5",
+        "concepts": [],
+        "prerequisites": [],
+        "status": "deprecated"
+      },
+      {
+        "slug": "numbers",
+        "name": "numbers",
+        "uuid": "e4107bd3-383e-47f7-a00b-f0cfb3b6126b",
+        "concepts": [],
+        "prerequisites": [],
+        "status": "deprecated"
+      }
+    ],
     "practice": [
       {
         "slug": "hello-world",

exercises/concept/futures/.docs/hints.md

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "lib/futures.dart"
+    ],
+    "test": [
+      "test/futures_test.dart"
+    ],
+    "exemplar": [
+      ".meta/exemplar.dart"
+    ]
+  },
+  "blurb": "[deprecated concept exercise]"
+}