Add FieldMask.empty().

Author: ehsannas

packages/firestore/src/local/local_documents_view.ts

@@ -217,7 +217,7 @@ export class LocalDocumentsView {
           batch.keys().forEach(key => {
             let mask: FieldMask | null = masks.has(key)
               ? masks.get(key)!
-              : new FieldMask([]);
+              : FieldMask.empty();
             mask = batch.applyToLocalViewWithFieldMask(docs.get(key)!, mask);
             masks.set(key, mask);
             if (documentsByBatchId.get(batch.batchId) === null) {

packages/firestore/src/model/field_mask.ts

@@ -42,6 +42,10 @@ export class FieldMask {
     );
   }
 
+  static empty(): FieldMask {
+    return new FieldMask([]);
+  }
+
   /**
    * Verifies that `fieldPath` is included by at least one field in this field
    * mask.

packages/firestore/src/model/mutation_batch.ts

@@ -101,8 +101,7 @@ export class MutationBatch {
    * @returns A `FieldMask` representing all the fields that are mutated.
    */
   applyToLocalView(document: MutableDocument): FieldMask | null {
-    const mutatedFields = new FieldMask([]);
-    return this.applyToLocalViewWithFieldMask(document, mutatedFields);
+    return this.applyToLocalViewWithFieldMask(document, FieldMask.empty());
   }
 
   /**