Fix flutter root exception

[jcollins-g]

Fixes #1705. Produces a much nicer error message when FLUTTER_ROOT is not set:

~/dart/dartdoc/testing/test_package_flutter_plugin
$ dart ../../bin/dartdoc.dart
Generating documentation for 'test_package_flutter_plugin' into /usr/local/google/home/jcollins/dart/dartdoc/testing/test_package_flutter_plugin/doc/api/
parsing lib/testlib.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/collection/collection.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/developer/developer.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/internal/internal.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/io/io.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/isolate/isolate.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/async/async.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/convert/convert.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/typed_data/typed_data.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/_empty.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/math/math.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/core/core.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/_http/http.dart...
parsing /usr/local/google/home/jcollins/.dartdoc_grinder/cleanFlutter/bin/cache/pkg/sky_engine/lib/ui/ui.dart...

Generation failed: Top level package requires Flutter but FLUTTER_ROOT environment variable not set.

[pq]

👍