Include field name

Author: schmidt-sebastian

packages/firestore/src/model/mutation.ts

@@ -50,7 +50,8 @@ export class FieldMask {
     fields.sort(FieldPath.comparator);
     debugAssert(
       !fields.some((v, i) => i !== 0 && v.isEqual(fields[i - 1])),
-      'FieldMask contains fields that are not unique'
+      'FieldMask contains field that is not unique: ' +
+        fields.find((v, i) => i !== 0 && v.isEqual(fields[i - 1]))!
     );
   }
 

packages/firestore/test/unit/util/misc.test.ts

@@ -48,7 +48,7 @@ describe('typeCast', () => {
 describe('FieldMask', () => {
   it('cannot contain duplicate fields', () => {
     expect(() => mask('a', 'b', 'a')).to.throw(
-      'FieldMask contains fields that are not unique'
+      'FieldMask contains field that is not unique: a'
     );
   });
 });