Add tests for internal apis used by development platform plugins (#6140)

Add test cases for internal apis used by development platform plugins so
we don't break them by accident during refactoring.

We should consider making the internal api more explicit.

firebase-crashlytics/src/androidTest/java/com/google/firebase/crashlytics/CrashlyticsPluginsTest.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.firebase.crashlytics;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import androidx.test.core.app.ApplicationProvider;
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.FirebaseOptions;
+import com.google.firebase.crashlytics.internal.common.CrashlyticsCore;
+import com.google.firebase.crashlytics.internal.common.DataCollectionArbiter;
+import java.lang.reflect.Field;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+/** Tests for the internal apis that development platform plugins call. */
+public class CrashlyticsPluginsTest {
+  private static final String APP_ID = "1:1:android:1a";
+  private static final String API_KEY = "API-KEY-API-KEY-API-KEY-API-KEY-API-KEY";
+  private static final String PROJECT_ID = "PROJECT-ID";
+
+  @Before
+  public void setUp() {
+    FirebaseApp.initializeApp(
+        ApplicationProvider.getApplicationContext(),
+        new FirebaseOptions.Builder()
+            .setApplicationId(APP_ID)
+            .setApiKey(API_KEY)
+            .setProjectId(PROJECT_ID)
+            .build());
+  }
+
+  @After
+  public void tearDown() {
+    FirebaseApp.clearInstancesForTest();
+  }
+
+  @Test
+  public void accessCrashlyticsCore() {
+    // Both Flutter and Unity plugins access CrashlyticsCore from FirebaseCrashlytics.core field.
+    CrashlyticsCore core = FirebaseCrashlytics.getInstance().core;
+    assertThat(core).isNotNull();
+
+    // Verify the internal method logFatalException exists without reflection.
+    Runnable logFatalException = () -> core.logFatalException(new Throwable());
+    assertThat(logFatalException).isNotNull();
+
+    // Verify the internal method setInternalKey exists without reflection.
+    Runnable setInternalKey = () -> core.setInternalKey("", "");
+    assertThat(setInternalKey).isNotNull();
+  }
+
+  @Test
+  public void accessDataCollection() throws Exception {
+    // The Unity plugin accesses CrashlyticsCore.dataCollectionArbiter this via reflection.
+    CrashlyticsCore core = FirebaseCrashlytics.getInstance().core;
+    Field field = core.getClass().getDeclaredField("dataCollectionArbiter");
+    field.setAccessible(true); // The dataCollectionArbiter field is private in CrashlyticsCore.
+    DataCollectionArbiter dataCollectionArbiter = (DataCollectionArbiter) field.get(core);
+
+    assertThat(dataCollectionArbiter).isNotNull();
+  }
+}

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/FirebaseCrashlytics.java

@@ -172,8 +172,8 @@ public class FirebaseCrashlytics {
     return new FirebaseCrashlytics(core);
   }
 
-  @VisibleForTesting // accessible for smoke tests
-  final CrashlyticsCore core;
+  /** Accessible for smoke tests, Unity, and Flutter plugins. */
+  @VisibleForTesting final CrashlyticsCore core;
 
   private FirebaseCrashlytics(@NonNull CrashlyticsCore core) {
     this.core = core;

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/common/CrashlyticsCore.java

@@ -74,7 +74,10 @@ public class CrashlyticsCore {
 
   private final Context context;
   private final FirebaseApp app;
+
+  /** This field is accessed by the Unity plugin via reflection. */
   private final DataCollectionArbiter dataCollectionArbiter;
+
   private final OnDemandCounter onDemandCounter;
 
   private final long startTime;