fix flutter plugin check (#1985)

diff --git a/lib/src/rules/avoid_web_libraries_in_flutter.dart b/lib/src/rules/avoid_web_libraries_in_flutter.dart
index a2ad24e..42b1da9 100644
--- a/lib/src/rules/avoid_web_libraries_in_flutter.dart
+++ b/lib/src/rules/avoid_web_libraries_in_flutter.dart
@@ -88,7 +88,8 @@
     }
 
     // Check for a web plugin context declaration.
-    return ((parsedPubspec['flutter'] ?? const {})['plugin'] ??
+    return (((parsedPubspec['flutter'] ?? const {})['plugin'] ??
+                const {})['platforms'] ??
             const {})['web'] ==
         null;
   }
diff --git a/test/_data/avoid_web_libraries_in_flutter/web_plugin/pubspec.yaml b/test/_data/avoid_web_libraries_in_flutter/web_plugin/pubspec.yaml
index 1a55a4b..8a3a36c 100644
--- a/test/_data/avoid_web_libraries_in_flutter/web_plugin/pubspec.yaml
+++ b/test/_data/avoid_web_libraries_in_flutter/web_plugin/pubspec.yaml
@@ -15,6 +15,7 @@
 
 flutter:
   plugin:
-    web:
-      pluginClass: SamplePlugin
-      fileName: main.dart
+    platforms:
+      web:
+        pluginClass: SamplePlugin
+        fileName: main.dart