Refactor out _handleImageInsert method

lib/models/documents/document.dart

@@ -173,9 +173,15 @@ class Document {
     for (int i = 0; i < ops.length; i++) {
       Operation op = ops[i];
       res.push(op);
-      bool nextOpIsImage = i + 1 < ops.length &&
-          ops[i + 1].isInsert &&
-          ops[i + 1].data is! String;
+      _handleImageInsert(i, ops, op, res);
+    }
+    return res;
+  }
+
+  static void _handleImageInsert(
+      int i, List<Operation> ops, Operation op, Delta res) {
+    bool nextOpIsImage =
+        i + 1 < ops.length && ops[i + 1].isInsert && ops[i + 1].data is! String;
     if (nextOpIsImage && !(op.data as String).endsWith('\n')) {
       res.push(Operation.insert('\n', null));
     }
@@ -190,8 +196,6 @@ class Document {
       res.push(Operation.insert('\n', null));
     }
   }
-    return res;
-  }
 
   Object _normalize(Object data) {
     if (data is String) {