Fix failing test

test/src/notebook/output-area/model.spec.ts

@@ -97,7 +97,7 @@ describe('notebook/output-area/model', () => {
           expect(args.oldIndex).to.be(-1);
           expect(args.newIndex).to.be(0);
           expect(args.oldValue).to.be(void 0);
-          expect(deepEqual(args.newValue, DEFAULT_OUTPUTS[0]));
+          expect(deepEqual(args.newValue as nbformat.IOutput, DEFAULT_OUTPUTS[0]));
           called = true;
         });
         model.add(DEFAULT_OUTPUTS[0]);