[flutter_tools] Fix null check in parsing web plugin from pubspec.yaml (#117939)

* fix null check in parsing web plugin yaml

* revert accidental diff

* remove comment

packages/flutter_tools/lib/src/platform_plugins.dart

@@ -541,7 +541,12 @@ class WebPlugin extends PluginPlatform {
   });
 
   factory WebPlugin.fromYaml(String name, YamlMap yaml) {
-    assert(validate(yaml));
+    if (yaml['pluginClass'] is! String) {
+      throwToolExit('The plugin `$name` is missing the required field `pluginClass` in pubspec.yaml');
+    }
+    if (yaml['fileName'] is! String) {
+      throwToolExit('The plugin `$name` is missing the required field `fileName` in pubspec.yaml');
+    }
     return WebPlugin(
       name: name,
       pluginClass: yaml['pluginClass'] as String,
@@ -549,13 +554,6 @@ class WebPlugin extends PluginPlatform {
     );
   }
 
-  static bool validate(YamlMap yaml) {
-    if (yaml == null) {
-      return false;
-    }
-    return yaml['pluginClass'] is String && yaml['fileName'] is String;
-  }
-
   static const String kConfigKey = 'web';
 
   /// The name of the plugin.

packages/flutter_tools/test/general.shard/plugin_parsing_test.dart

@@ -290,6 +290,29 @@ void main() {
     ]);
   });
 
+  testWithoutContext('Web plugin tool exits if fileName field missing', () {
+    final FileSystem fileSystem = MemoryFileSystem.test();
+    const String pluginYamlRaw =
+      'platforms:\n'
+      ' web:\n'
+      '  pluginClass: WebSamplePlugin\n';
+
+    final YamlMap pluginYaml = loadYaml(pluginYamlRaw) as YamlMap;
+    expect(
+      () => Plugin.fromYaml(
+        _kTestPluginName,
+        _kTestPluginPath,
+        pluginYaml,
+        null,
+        const <String>[],
+        fileSystem: fileSystem,
+      ),
+      throwsToolExit(
+        message: 'The plugin `$_kTestPluginName` is missing the required field `fileName` in pubspec.yaml',
+      ),
+    );
+  });
+
   testWithoutContext('Windows assumes win32 when no variants are given', () {
     final FileSystem fileSystem = MemoryFileSystem.test();
     const String pluginYamlRaw =