目前公司项目从纯iOS、Android端改为混合开发，需要在原生项目中嵌入flutter开发的代码，我当前的flutter版本是 v1.12.13+hotfix.9-stable，使用过程中有些问题，于是在项目上线后，对flutter做了升级处理，升级到 v1.20.2，升级后运行项目报错如下：

Xcode's output:
↳
    ../../../../../flutter/flutter_sdk/.pub-cache/hosted/pub.flutter-io.cn/flutter_datetime_picker-1.3.8/lib/src/datetime_picker_theme.dart:6:28: Error: Type
    'DiagnosticableMixin' not found.
    class DatePickerTheme with DiagnosticableMixin {
                               ^^^^^^^^^^^^^^^^^^^
    ../../../../../flutter/flutter_sdk/.pub-cache/hosted/pub.flutter-io.cn/flutter_datetime_picker-1.3.8/lib/src/datetime_picker_theme.dart:6:7: Error: The type
    'DiagnosticableMixin' can't be mixed in.
    class DatePickerTheme with DiagnosticableMixin {
          ^

    Command PhaseScriptExecution failed with a nonzero exit code
    note: Using new build system
    note: Building targets in parallel
    note: Planning build
    note: Constructing build description

Could not build the precompiled application for the device.

原因是flutter项目中用到了时间选择器，并设置了主题DatePickerTheme，查看DatePickerTheme源码发现在有一段官方注释如下：

// Migrate DiagnosticableMixin to Diagnosticable until
// https://github.com/flutter/flutter/pull/51495 makes it into stable (v1.15.21)
class DatePickerTheme with DiagnosticableMixin {

DatePickerTheme混入的DiagnosticableMixin在flutter v1.15.21版本已经发生了变更，由DiagnosticableMixin改为了Diagnosticable。

解决方案

我们可以直接用 Diagnosticable 替换 DiagnosticableMixin，如下：

// Migrate DiagnosticableMixin to Diagnosticable until
// https://github.com/flutter/flutter/pull/51495 makes it into stable (v1.15.21)
class DatePickerTheme with Diagnosticable {

重新运行，成功！