Let docgen detect a dart-sdk directory that's differently named

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..e082d49dac733a5af21a1013db73da4c93912dfc 100644
--- a/pkg/docgen/lib/src/package_helpers.dart
+++ b/pkg/docgen/lib/src/package_helpers.dart
@@ -23,10 +23,17 @@ String get rootDirectory {
   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') {
+  var found = false;
+  // When we find a version file or sdk we are one level below the root.
+  while (!found && base != 'sdk' && base != 'pkg') {
     root = path.dirname(root);
     base = path.basename(root);
+    // Look for something that looks like the dart-sdk directory, which we
+    // expect to be in the path above us and to have a 'version' file containing
+    // the same version as we are running.
+    if (hasMatchingVersionFile(root)) {
+      found = true;
+    }
     if (root == base) {
       // We have reached the root of the filesystem without finding anything.
       throw new FileSystemException("Cannot find SDK directory starting from ",
@@ -38,6 +45,17 @@ String get rootDirectory {
 }
 String _rootDirectoryCache;
 
+
+/// Does this directory contain a version file that has the same version as
+/// we do.
+bool hasMatchingVersionFile(String root) {
+  var versionFile = new File(path.join(root, 'version'));
+  if (!versionFile.existsSync()) return false;
+  var version = versionFile.readAsStringSync().trim();
+  var vmVersion = Platform.version;
+  return vmVersion.startsWith(version);
+}
+
 /// Given a LibraryMirror that is a library, return the name of the directory
 /// holding the package information for that library. If the library is not
 /// part of a package, return null.