Let docgen detect a dart-sdk directory that's differently named, and do it correctly on bleeding ed…

pkg/docgen/lib/src/package_helpers.dart

Index: pkg/docgen/lib/src/package_helpers.dart
diff --git a/pkg/docgen/lib/src/package_helpers.dart b/pkg/docgen/lib/src/package_helpers.dart
index ca189fa15c08d64b463e791d02f2a1805d744819..f437166e6e2d5d7b4073219bb92e24f95233b268 100644
--- a/pkg/docgen/lib/src/package_helpers.dart
+++ b/pkg/docgen/lib/src/package_helpers.dart
@@ -12,19 +12,19 @@ import 'package:path/path.dart' as path;
 import 'package:yaml/yaml.dart';
 
 /// Helper accessor to determine the full pathname of the root of the dart
-/// checkout. We can be in one of three situations:
-/// 1) Running from pkg/docgen/bin/docgen.dart
-/// 2) Running from a snapshot in a build,
-///   e.g. xcodebuild/ReleaseIA32/dart-sdk/bin
-/// 3) Running from a built distribution,
-///   e.g. ...somename/dart-sdk/bin/snapshots
+/// checkout if we are running without the --sdk parameter specified. That
+/// normally means we are running directly from source, and we expect to
+/// find the root as the directory above 'pkg'. The only other time this
+/// would happen is running a snapshot directly, rather than from the
+/// dartdocgen script, where we look for the dart-sdk directory. If not
+/// using the script and not in a normal directory structure, you'll need
+/// to pass the --sdk parameter.
 String get rootDirectory {
   if (_rootDirectoryCache != null) return _rootDirectoryCache;
   var scriptDir = path.absolute(path.dirname(Platform.script.toFilePath()));
   var root = scriptDir;
   var base = path.basename(root);
-  // When we find dart-sdk or sdk we are one level below the root.
-  while (base != 'dart-sdk' && base != 'sdk' && base != 'pkg') {
+  while (base != 'dart-sdk' && base != 'pkg') {
     root = path.dirname(root);
     base = path.basename(root);
     if (root == base) {