[CQ] fix nullability problems for flutter/analytics (#8294)

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

See #8291.

If we pursue #8292, we
could mark `src/io/flutter/analytics/` 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.
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best practices
([discussion](#8098)).
</details>

Author: pq

flutter-idea/src/io/flutter/analytics/UnifiedAnalytics.java

@@ -70,9 +70,9 @@ public void manageConsent() {
     }
   }
 
-  private CompletableFuture<JsonObject> makeUnifiedAnalyticsRequest(String requestName,
-                                                                    @NotNull DartToolingDaemonService service,
-                                                                    @NotNull JsonObject params) {
+  private @NotNull CompletableFuture<JsonObject> makeUnifiedAnalyticsRequest(String requestName,
+                                                                             @NotNull DartToolingDaemonService service,
+                                                                             @NotNull JsonObject params) {
     CompletableFuture<JsonObject> finalResult = new CompletableFuture<>();
     try {
       service.sendRequest("UnifiedAnalytics." + requestName, params, false, object -> {
@@ -90,7 +90,7 @@ private CompletableFuture<JsonObject> makeUnifiedAnalyticsRequest(String request
     return finalResult;
   }
 
-  private @Nullable CompletableFuture<Boolean> telemetryEnabled(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
+  private @NotNull CompletableFuture<Boolean> telemetryEnabled(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
     return makeUnifiedAnalyticsRequest("telemetryEnabled", service, params).thenCompose(result -> {
       assert result != null;
       JsonPrimitive value = result.getAsJsonPrimitive("value");
@@ -104,7 +104,7 @@ private CompletableFuture<JsonObject> makeUnifiedAnalyticsRequest(String request
     });
   }
 
-  private @Nullable CompletableFuture<Boolean> clientShowedMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
+  private @NotNull CompletableFuture<Boolean> clientShowedMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
     return makeUnifiedAnalyticsRequest("clientShowedMessage", service, params).thenCompose(result -> {
       assert result != null;
       JsonPrimitive type = result.getAsJsonPrimitive("type");
@@ -118,9 +118,9 @@ private CompletableFuture<JsonObject> makeUnifiedAnalyticsRequest(String request
     });
   }
 
-  private @Nullable CompletableFuture<Boolean> setTelemetry(@NotNull DartToolingDaemonService service,
-                                                            @NotNull JsonObject params,
-                                                            Boolean canSendAnalytics) {
+  private @NotNull CompletableFuture<Boolean> setTelemetry(@NotNull DartToolingDaemonService service,
+                                                           @NotNull JsonObject params,
+                                                           Boolean canSendAnalytics) {
     params.addProperty("enable", canSendAnalytics);
     return makeUnifiedAnalyticsRequest("setTelemetry", service, params).thenCompose(result -> {
       assert result != null;
@@ -178,7 +178,7 @@ else if ("Android-Studio".equals(ideValue)) {
     }
   }
 
-  private @Nullable CompletableFuture<Boolean> shouldShowMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
+  private @NotNull CompletableFuture<Boolean> shouldShowMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
     return makeUnifiedAnalyticsRequest("shouldShowMessage", service, params).thenCompose(result -> {
       assert result != null;
       JsonPrimitive value = result.getAsJsonPrimitive("value");
@@ -192,7 +192,7 @@ else if ("Android-Studio".equals(ideValue)) {
     });
   }
 
-  private @Nullable CompletableFuture<String> getConsentMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
+  private @NotNull CompletableFuture<String> getConsentMessage(@NotNull DartToolingDaemonService service, @NotNull JsonObject params) {
     return makeUnifiedAnalyticsRequest("getConsentMessage", service, params).thenCompose(result -> {
       assert result != null;
       JsonPrimitive value = result.getAsJsonPrimitive("value");

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

@@ -12,7 +12,7 @@
 import java.util.concurrent.CompletableFuture;
 
 public class DtdUtils {
-  public CompletableFuture<DartToolingDaemonService> readyDtdService(@NotNull Project project) {
+  public @NotNull CompletableFuture<DartToolingDaemonService> readyDtdService(@NotNull Project project) {
     final DartToolingDaemonService dtdService = DartToolingDaemonService.getInstance(project);
     CompletableFuture<DartToolingDaemonService> readyService = new CompletableFuture<>();
     int attemptsRemaining = 10;