How to use the text-buffer.deserialize function in text-buffer

const handleBufferState = bufferState => {
      if (bufferState.shouldDestroyOnFileDelete == null) {
        bufferState.shouldDestroyOnFileDelete = () =>
          atom.config.get('core.closeDeletedFileTabs');
      }

      // Use a little guilty knowledge of the way TextBuffers are serialized.
      // This allows TextBuffers that have never been saved (but have filePaths) to be deserialized, but prevents
      // TextBuffers backed by files that have been deleted from being saved.
      bufferState.mustExist = bufferState.digestWhenLastPersisted !== false;

      return TextBuffer.deserialize(bufferState).catch(_ => {
        this.retiredBufferIDs.add(bufferState.id);
        this.retiredBufferPaths.add(bufferState.filePath);
        return null;
      });
    };

it('persists editors\' grammar overrides', async () => {
      const buffer1 = new TextBuffer()
      const buffer2 = new TextBuffer()

      grammarRegistry.loadGrammarSync(require.resolve('language-c/grammars/c.cson'))
      grammarRegistry.loadGrammarSync(require.resolve('language-html/grammars/html.cson'))
      grammarRegistry.loadGrammarSync(require.resolve('language-javascript/grammars/javascript.cson'))

      grammarRegistry.maintainLanguageMode(buffer1)
      grammarRegistry.maintainLanguageMode(buffer2)
      grammarRegistry.assignLanguageMode(buffer1, 'source.c')
      grammarRegistry.assignLanguageMode(buffer2, 'source.js')

      const buffer1Copy = await TextBuffer.deserialize(buffer1.serialize())
      const buffer2Copy = await TextBuffer.deserialize(buffer2.serialize())

      const grammarRegistryCopy = new GrammarRegistry({config: atom.config})
      grammarRegistryCopy.deserialize(JSON.parse(JSON.stringify(grammarRegistry.serialize())))

      grammarRegistryCopy.loadGrammarSync(require.resolve('language-c/grammars/c.cson'))
      grammarRegistryCopy.loadGrammarSync(require.resolve('language-html/grammars/html.cson'))

      expect(buffer1Copy.getLanguageMode().getLanguageId()).toBe(null)
      expect(buffer2Copy.getLanguageMode().getLanguageId()).toBe(null)

      grammarRegistryCopy.maintainLanguageMode(buffer1Copy)
      grammarRegistryCopy.maintainLanguageMode(buffer2Copy)
      expect(buffer1Copy.getLanguageMode().getLanguageId()).toBe('source.c')
      expect(buffer2Copy.getLanguageMode().getLanguageId()).toBe(null)

      grammarRegistryCopy.loadGrammarSync(require.resolve('language-javascript/grammars/javascript.cson'))