Add redirect for 2.4 SDK changelog (#1753)

Fixes #1752

Author: kwalrath

firebase.json

@@ -194,7 +194,8 @@
       { "source": "/tos@(|.htm|.html)", "destination": "/terms", "type": 301 },
       { "source": "/web-ui/observables{,/**}", "destination": "https://pub.dev/packages/observable", "type": 301 },
       { "source": "/dart-1", "destination": "https://v1-dartlang-org.firebaseapp.com", "type": 301 },
-      { "source": "/packages", "destination": "https://pub.dev", "type": 301 }
+      { "source": "/packages", "destination": "https://pub.dev", "type": 301 },
+      { "source": "/sdk-changelog-2.4.0", "destination": "https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#240---2019-06-27", "type": 301 }
     ]
   }
 }