1. 问题描述

Downloading Dart SDK from Flutter engine 4e54bc93ca9aaf2156fb06266c9fe509e8599a5f...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:05:00 --:--:--     0
curl: (56) Proxy CONNECT aborted due to timeout

Failed to retrieve the Dart SDK from: https://mirrors.sjtug.sjtu.edu.cn/flutter_infra/flutter/4e54bc93ca9aaf2156fb06266c9fe509e8599a5f/dart-sdk-darwin-x64.zip
If you're located in China, please see this page:
  https://flutter.io/community/china

这里使用的https://mirrors.sjtug.sjtu.edu.cn这个源，依旧下载不了。

2. 解决方案

下载dart-sdk：wget https://mirrors.sjtug.sjtu.edu.cn/flutter_infra/flutter/4e54bc93ca9aaf2156fb06266c9fe509e8599a5f/dart-sdk-darwin-x64.zip。找到安装目录，我这里是/Users/zhenlei/Library/flutter/，把dart-sdk-darwin-x64.zip放到bin/cache/下，请确保文件名正确。
打开/bin/internal/update_dart_sdk.sh文件，注释掉以下行

  rm -rf -- "$DART_SDK_PATH"
  mkdir -m 755 -p -- "$DART_SDK_PATH"
  
  curl --continue-at - --location --output "$DART_SDK_ZIP" "$DART_SDK_URL" 2>&1 || {
    echo
    echo "Failed to retrieve the Dart SDK from: $DART_SDK_URL"
    echo "If you're located in China, please see this page:"
    echo "  https://flutter.io/community/china"
    echo
    rm -f -- "$DART_SDK_ZIP"
    exit 1
  }