Added react-is to validate the proptype for Route's component. (#6417)

* Added react-is to validate the proptype for Route's component.

Removed a couple of outdated TODOs that dealt with __DEV__

The rest of the changes were done just from an npm install command

React-is does add a tiny bit of size ( https://bundlephobia.com/result?p=react-is@16.5.2 ) as noted by the snapshots, but it seems like the best (and official) way to support the check.

* Revert removal of __DEV__

* Updated the error message to be a little more clear about which prop is invalid.

* Realized it wasn't saying what I wanted, so I just reverted it.

packages/react-router-dom/.size-snapshot.json

@@ -1,8 +1,8 @@
 {
   "esm/react-router-dom.js": {
-    "bundled": 7935,
-    "minified": 4844,
-    "gzipped": 1607,
+    "bundled": 7968,
+    "minified": 4870,
+    "gzipped": 1613,
     "treeshaked": {
       "rollup": {
         "code": 1240,
@@ -14,13 +14,13 @@
     }
   },
   "umd/react-router-dom.js": {
-    "bundled": 161801,
-    "minified": 58038,
-    "gzipped": 16043
+    "bundled": 171152,
+    "minified": 61992,
+    "gzipped": 16916
   },
   "umd/react-router-dom.min.js": {
-    "bundled": 101990,
-    "minified": 36745,
-    "gzipped": 10247
+    "bundled": 106635,
+    "minified": 38911,
+    "gzipped": 11286
   }
 }