anchor.diff:  
       test('two currentTransaction', () {
         db.transaction((txn) {
  -        expect(db.currentTransaction.id, 1);
  +        expect(db.currentTransaction.id, transactionIdAfterOpen + 1);
         }).then((_) {
           // expect(db.currentTransaction, null);
         });
         return db.transaction((txn) {
  -        expect(db.currentTransaction.id, 2);
  +        expect(db.currentTransaction.id, transactionIdAfterOpen + 2);
         }).then((_) {
           // expect(db.currentTransaction, null);
         });
  
positive.code: 
      test('two currentTransaction follow', () {
        db.transaction((txn) {
          expect(db.currentTransaction.id, 1);
        }).then((_) {
          expect(db.currentTransaction, null);
          return db.transaction((txn) {
            expect(db.currentTransaction.id, 2);
          }).then((_) {
            expect(db.currentTransaction, null);
          });
  
negative.code:       });
      });
  
      int transactionIdAfterOpen = 1;
      test('one currentTransaction', () {
        db.transaction((txn) {
          expect(db.currentTransaction.id, transactionIdAfterOpen + 1);
          return Future.value().then((_) {
            expect(db.currentTransaction.id, transactionIdAfterOpen + 1);
          }).then((_) {
            expect(db.currentTransaction.id, transactionIdAfterOpen + 1);
          });
        }).then((_) {
          expect(db.currentTransaction, null);
  