首先启用dartdoc:
flutter pub global activate dartdoc
然后执行dartdoc命令
dartdoc

执行dartdoc命令后，出现如下错误：

#0      _validateArgList (package:path/src/context.dart:1101:5)
#1      Context.join (package:path/src/context.dart:240:5)
#2      join (package:path/path.dart:262:13)
#3      createDartdocOptions.<anonymous closure> (package:dartdoc/src/dartdoc_options.dart:1587:16)
#4      DartdocSyntheticOption._valueAtFromSynthetic (package:dartdoc/src/dartdoc_options.dart:798:44)
#5      DartdocOptionArgSynth.valueAt (package:dartdoc/src/dartdoc_options.dart:756:12)
#6      DartdocOptionContext.sdkDir (package:dartdoc/src/dartdoc_options.dart:1405:44)
#7      PubPackageBuilder.sdk (package:dartdoc/src/model/package_builder.dart:81:60)
#8      PubPackageBuilder.buildPackageGraph (package:dartdoc/src/model/package_builder.dart:67:7)
#9      Dartdoc.generateDocsBase (package:dartdoc/dartdoc.dart:175:41)
#10     Dartdoc.generateDocs (package:dartdoc/dartdoc.dart:216:34)
#11     _rootRun (dart:async/zone.dart:1184:13)
#12     _CustomZone.run (dart:async/zone.dart:1077:19)
#13     _runZoned (dart:async/zone.dart:1619:10)
#14     runZonedGuarded (dart:async/zone.dart:1608:12)
#15     Dartdoc.executeGuarded (package:dartdoc/dartdoc.dart:475:5)
#16     main (file:///Users/chongyang/Library/flutter/.pub-cache/hosted/pub.flutter-io.cn/dartdoc-0.32.1/bin/dartdoc.dart:24:11)
<asynchronous suspension>
#17     _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:299:32)
#18     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

搜索到原因为未设置FLUTTER_ROOT到环境变量中。（在我的系统中设置的变量名是FLUTTER_HOME,推断dartdoc只查找默认值是FLUTTER_ROOT的环境变量）。
将FLUTTER_ROOT添加到环境变量后，文档生成成功。（如何添加环境变量，请出门左转问度娘）
生成的文档在[项目根目录]/doc/api下。