Generate docs fails for flutter packages when FLUTTER_ROOT is not set

[trevorwang]

https://pub.dartlang.org/documentation/fzxing/0.1.0+2/log.txt

STDERR:

Generation failed: Invalid argument(s): join(null, "bin", "cache", "dart-sdk"): part 0 was null, but part 1 was not.
package:path/src/context.dart 1088                                           _validateArgList
package:path/src/context.dart 231                                            Context.join
package:path/path.dart 257                                                   join
package:dartdoc/src/dartdoc_options.dart 1114                                createDartdocOptions.<fn>
package:dartdoc/src/dartdoc_options.dart 421                                 DartdocOption&&DartdocSyntheticOption._valueAtFromSynthetic
package:dartdoc/src/dartdoc_options.dart 382                                 DartdocOptionArgSynth.valueAt
package:dartdoc/src/dartdoc_options.dart 948                                 DartdocOptionContext.sdkDir
package:dartdoc/src/package_meta.dart 90                                     new PackageMeta.fromElement
package:dartdoc/src/model.dart 5488                                          PackageBuilder.processLibrary

[jcollins-g]

This exception should be improved -- the root cause is that FLUTTER_ROOT was not set in the environment.

@isoos , could this be a problem with how the pub site is running dartdoc? Without FLUTTER_ROOT dartdoc can't figure out how to link to flutter for packages using it.

[isoos]

We were always setting the FLUTTER_SDK variable.
Should we set FLUTTER_ROOT as well, or only the later?

[jcollins-g]

For Dartdoc, the FLUTTER_ROOT environment variable must be set; we don't directly use FLUTTER_SDK.

[jcollins-g]

@isoos I'm not sure FLUTTER_SDK is a thing; searches in the flutter repository for this string come up empty.