deps: Add flutter_checks and legacy_checks

Thanks lishaduck for creating these with us in mind! Discussion:
  zulip#232 (comment)

pubspec.lock

@@ -420,6 +420,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_checks:
+    dependency: "direct main"
+    description:
+      name: flutter_checks
+      sha256: b0a40b15d38436b7c08b83353e74b0569c1ec687bd81c9556091fbb8807c1b99
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.1.1"
   flutter_color_models:
     dependency: "direct main"
     description:
@@ -683,6 +691,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.1"
+  legacy_checks:
+    dependency: "direct main"
+    description:
+      name: legacy_checks
+      sha256: b22e5b1ff55a14e8bd91acafbabff909e14829b73ca492dcdaf0fbbb70476079
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.1.0"
   lints:
     dependency: transitive
     description:

pubspec.yaml

@@ -65,6 +65,8 @@ dependencies:
   video_player: ^2.8.3
   zulip_plugin:
     path: ./packages/zulip_plugin
+  flutter_checks: ^0.1.1
+  legacy_checks: ^0.1.0
   # Keep list sorted when adding dependencies; it helps prevent merge conflicts.
 
 dependency_overrides: