1、打开终端

cd 到项目路径下
然后终端输入

flutter packages pub global activate webdev

提示下面内容，警告去path环境变量文件里

Downloading source_maps 0.10.9...
Downloading collection 1.14.13...
Downloading matcher 0.12.9...
Downloading stack_trace 1.9.6...
Precompiling executables...
Precompiled webdev:webdev.
Installed executable webdev.
Warning: Pub installs executables into /opt/flutter/.pub-cache/bin, which is not on your path.
You can fix that by adding this to your shell's config file (.bashrc, .bash_profile, etc.):

  export PATH="$PATH":"/opt/flutter/.pub-cache/bin"

Activated webdev 2.7.1.

2、然后cd 到项目 flutter packages upgrade

终端再输入

flutter packages upgrade

提示警告如下，可忽略

Warning: You are using these overridden dependencies:                   
! flutter_web 0.0.0 from path ../../packages/flutter_web                
! flutter_web_ui 0.0.0 from path ../../packages/flutter_web_ui          
Running "flutter pub upgrade" in hello_world...

3、接下来输入 webdev serve

webdev serve

提示 -bash: webdev: command not found 这个时候解决第一步问题。
找到.bash_profile 文件打开
把之前的 export PATH="$PATH":"/opt/flutter/.pub-cache/bin" 拷过来

并且配置

export PUB_HOSTED_URL=https://pub.flutter-io.cn
export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn
export PATH=/opt/flutter/bin/cache/dart-sdk/bin:$PATH
export PATH=/opt/flutter/bin:$PATH

接下来输入

webdev serve

提示如下

webdev could not run for this project.
No pubspec.lock file found, please run "pub get" first.

然后输入

pub get

4、接下来再输入 webdev serve

提示如下

Downloading source_maps 0.10.9...
Downloading archive 2.0.13...
Downloading quiver 2.1.5...
Downloading dart_style 1.3.9...
Downloading intl 0.16.1...
Changed 65 dependencies!
hehuandeMacBook-Pro:spinning_square hehuan$ webdev serve
[INFO] Building new asset graph completed, took 1.3s
[INFO] Checking for unexpected pre-existing outputs. completed, took 1ms
[INFO] Serving `web` on http://127.0.0.1:8080
[INFO] Generating SDK summary completed, took 3.3s
[INFO] Running build completed, took 23.4s
[INFO] Caching finalized dependency graph completed, took 361ms
[INFO] Succeeded after 23.8s with 594 outputs (3161 actions)
[INFO] ------------------------------------------------------------------------
[INFO] Received request for entrypoint at http://127.0.0.1:8080/main.dart.bootst[INFO] Injected debugging metadata for entrypoint at http://127.0.0.1:8080/main.[INFO] Received request for entrypoint at http://localhost:8080/main.dart.bootst[INFO] Injected debugging metadata for entrypoint at http://localhost:8080/main.dart.bootstrap.js

看到http://127.0.0.1:8080/ 表示服务启动成功