SyncEngine.java: Add missing import of java.util.Iterator and change some return types to more general types (e.g. LinkedHashSet -> List).

Author: dconeybe

firebase-firestore/src/main/java/com/google/firebase/firestore/core/SyncEngine.java

@@ -58,6 +58,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
@@ -710,15 +711,15 @@ private void pumpEnqueuedLimboResolutions() {
   }
 
   @VisibleForTesting
-  public HashMap<DocumentKey, Integer> getActiveLimboDocumentResolutions() {
+  public Map<DocumentKey, Integer> getActiveLimboDocumentResolutions() {
     // Make a defensive copy as the Map continues to be modified.
-    return new HashMap(activeLimboTargetsByKey);
+    return new HashMap<>(activeLimboTargetsByKey);
   }
 
   @VisibleForTesting
-  public LinkedHashSet<DocumentKey> getEnqueuedLimboDocumentResolutions() {
+  public List<DocumentKey> getEnqueuedLimboDocumentResolutions() {
     // Make a defensive copy as the LinkedHashSet continues to be modified.
-    return new LinkedHashSet(enqueuedLimboResolutions);
+    return new ArrayList<>(enqueuedLimboResolutions);
   }
 
   public void handleCredentialChange(User user) {