[CQ] fix nullability problems for flutter/dart (#8300)

Fix nullability problems in `src/io/flutter/dart/`.

See #8291.

If we pursue #8292, we
could mark `src/io/flutter/dart/` as null-"clean".


---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

flutter-idea/src/io/flutter/dart/FlutterDartAnalysisServer.java

@@ -36,7 +36,7 @@ public class FlutterDartAnalysisServer implements Disposable {
    * Each key is a notification identifier.
    * Each value is the set of files subscribed to the notification.
    */
-  private final Map<String, List<String>> subscriptions = new HashMap<>();
+  private final @NotNull Map<String, List<String>> subscriptions = new HashMap<>();
 
   @VisibleForTesting
   protected final Map<String, List<FlutterOutlineListener>> fileOutlineListeners = new HashMap<>();

flutter-idea/src/io/flutter/dart/FlutterRequestUtilities.java

@@ -9,7 +9,6 @@
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonPrimitive;
-import org.dartlang.analysis.server.protocol.FlutterWidgetPropertyValue;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
@@ -29,30 +28,11 @@ public class FlutterRequestUtilities {
   private static final String SUBSCRIPTIONS = "subscriptions";
 
   private static final String METHOD_FLUTTER_SET_SUBSCRIPTIONS = "flutter.setSubscriptions";
-  private static final String METHOD_FLUTTER_GET_WIDGET_DESCRIPTION = "flutter.getWidgetDescription";
-  private static final String METHOD_FLUTTER_SET_WIDGET_PROPERTY_VALUE = "flutter.setWidgetPropertyValue";
 
   private FlutterRequestUtilities() {
     throw new AssertionError("No instances.");
   }
 
-  public static JsonObject generateFlutterGetWidgetDescription(String id, String file, int offset) {
-    final JsonObject params = new JsonObject();
-    params.addProperty(FILE, file);
-    params.addProperty(OFFSET, offset);
-    return buildJsonObjectRequest(id, METHOD_FLUTTER_GET_WIDGET_DESCRIPTION, params);
-  }
-
-  public static JsonObject generateFlutterSetWidgetPropertyValue(String id, int propertyId, @Nullable FlutterWidgetPropertyValue value) {
-    final JsonObject params = new JsonObject();
-    params.addProperty(ID, propertyId);
-    // An omitted value indicates that the property should be removed.
-    if (value != null) {
-      params.add(VALUE, value.toJson());
-    }
-    return buildJsonObjectRequest(id, METHOD_FLUTTER_SET_WIDGET_PROPERTY_VALUE, params);
-  }
-
   /**
    * Generate and return a {@value #METHOD_FLUTTER_SET_SUBSCRIPTIONS} request.
    * <p>
@@ -66,15 +46,15 @@ public static JsonObject generateFlutterSetWidgetPropertyValue(String id, int pr
    * }
    * </pre>
    */
-  public static JsonObject generateAnalysisSetSubscriptions(String id,
-                                                            Map<String, List<String>> subscriptions) {
+  public static JsonObject generateAnalysisSetSubscriptions(@NotNull String id,
+                                                            Map<String, @NotNull List<String>> subscriptions) {
     final JsonObject params = new JsonObject();
     params.add(SUBSCRIPTIONS, buildJsonElement(subscriptions));
     return buildJsonObjectRequest(id, METHOD_FLUTTER_SET_SUBSCRIPTIONS, params);
   }
 
   @NotNull
-  private static JsonElement buildJsonElement(Object object) {
+  private static JsonElement buildJsonElement(@Nullable Object object) {
     if (object instanceof Boolean) {
       return new JsonPrimitive((Boolean)object);
     }
@@ -87,14 +67,17 @@ else if (object instanceof String) {
     else if (object instanceof List<?> list) {
       final JsonArray jsonArray = new JsonArray();
       for (Object item : list) {
-        final JsonElement jsonItem = buildJsonElement(item);
-        jsonArray.add(jsonItem);
+        if (item != null) {
+          final JsonElement jsonItem = buildJsonElement(item);
+          jsonArray.add(jsonItem);
+        }
       }
       return jsonArray;
     }
     else if (object instanceof Map<?, ?> map) {
       final JsonObject jsonObject = new JsonObject();
       for (Map.Entry<?, ?> entry : map.entrySet()) {
+        if (entry == null) continue;
         final Object key = entry.getKey();
         // Prepare the string key.
         final String keyString;
@@ -114,10 +97,6 @@ else if (object instanceof Map<?, ?> map) {
     throw new IllegalArgumentException("Unable to convert to JSON: " + object);
   }
 
-  private static JsonObject buildJsonObjectRequest(String idValue, String methodValue) {
-    return buildJsonObjectRequest(idValue, methodValue, null);
-  }
-
   private static JsonObject buildJsonObjectRequest(String idValue, String methodValue,
                                                    JsonObject params) {
     final JsonObject jsonObject = new JsonObject();
@@ -133,7 +112,7 @@ private static JsonObject buildJsonObjectRequest(String idValue, String methodVa
    * Return the name of the given object, may be {@code "null"} String.
    */
   @NotNull
-  private static String getClassName(Object object) {
+  private static String getClassName(@Nullable Object object) {
     return object != null && object.getClass().getName() != null ? object.getClass().getName() : "null";
   }
 }