Docgen snapshot needs to use dart-sdk as its SDK root in a downloaded SDK

pkg/docgen/lib/docgen.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// **docgen** is a tool for creating machine readable representations of Dart
 /// code metadata, including: classes, members, comments and annotations.
 ///
 /// docgen is run on a `.dart` file or a directory containing `.dart` files.
 ///
 ///      $ dart docgen.dart [OPTIONS] [FILE/DIR]
@@ skipping 129 matching lines @@
   return result;
 }
 
 /// Analyzes set of libraries by getting a mirror system and triggers the
 /// documentation of the libraries.
 Future<MirrorSystem> getMirrorSystem(List<Uri> libraries,
     {String packageRoot, bool parseSdk: false}) {
   if (libraries.isEmpty) throw new StateError('No Libraries.');
 
   // Finds the root of SDK library based off the location of docgen.
+  // We have two different places to look, depending if we're in a development
+  // repo or in a built SDK, either sdk or dart-sdk respectively
   var root = _Generator._rootDirectory;
   var sdkRoot = path.normalize(path.absolute(path.join(root, 'sdk')));
+  if (!new Directory(sdkRoot).existsSync()) {
+    sdkRoot = path.normalize(path.absolute(path.join(root, 'dart-sdk')));
+  }
   _Generator.logger.info('SDK Root: ${sdkRoot}');
   return _Generator._analyzeLibraries(libraries, sdkRoot,
       packageRoot: packageRoot);
 }
 
 /// For types that we do not explicitly create or have not yet created in our
 /// entity map (like core types).
 class DummyMirror implements Indexable {
   DeclarationMirror mirror;
   /// The library that contains this element, if any. Used as a hint to help
@@ skipping 288 matching lines @@
   /// available on the file system.
   static void _ensureOutputDirectory(String outputDirectory, bool append) {
     _outputDirectory = outputDirectory;
     if (!append) {
       var dir = new Directory(_outputDirectory);
       if (dir.existsSync()) dir.deleteSync(recursive: true);
      }
   }
 
   /// Helper accessor to determine the full pathname of the root of the dart
-  /// checkout.
+  /// 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
   static String get _rootDirectory {
     var scriptDir = path.absolute(path.dirname(Platform.script.toFilePath()));
     var root = scriptDir;
-    while(path.basename(root) != 'dart') {
+    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') {
       root = path.dirname(root);
+      base = path.basename(root);
+      if (root == base) {
+        // We have reached the root of the filesystem without finding anything.
+        throw new FileSystemException(
+            "Cannot find SDK directory starting from ",
+            scriptDir);
+        }
     }
-    return root;
+    return path.dirname(root);
   }
 
   /// Analyzes set of libraries and provides a mirror system which can be used
   /// for static inspection of the source code.
   static Future<MirrorSystem> _analyzeLibraries(List<Uri> libraries,
         String libraryRoot, {String packageRoot}) {
     SourceFileProvider provider = new CompilerSourceFileProvider();
     api.DiagnosticHandler diagnosticHandler =
         (new FormattingDiagnosticHandler(provider)
           ..showHints = false
@@ skipping 1035 matching lines @@
 
   OwnedIndexable(DeclarationMirror mirror, this.owner) : super(mirror);
 
   /// Generates MDN comments from database.json.
   String _mdnComment() {
     //Check if MDN is loaded.
     if (Indexable._mdn == null) {
       // Reading in MDN related json file.
       var root = _Generator._rootDirectory;
       var mdnPath = path.join(root, 'utils/apidoc/mdn/database.json');
-      Indexable._mdn = JSON.decode(new File(mdnPath).readAsStringSync());
+      var mdnFile = new File(mdnPath);
+      if (mdnFile.existsSync()) {
+        Indexable._mdn = JSON.decode(mdnFile.readAsStringSync());
+      } else {
+        _Generator.logger.warning("Cannot find MDN docs expected at $mdnPath");
+        Indexable._mdn = {};
+      }
     }
     var domAnnotation = this.annotations.firstWhere(
         (e) => e.mirror.qualifiedName == 'metadata.DomName',
         orElse: () => null);
     if (domAnnotation == null) return '';
     var domName = domAnnotation.parameters.single;
     var parts = domName.split('.');
     if (parts.length == 2) return _mdnMemberComment(parts[0], parts[1]);
     if (parts.length == 1) return _mdnTypeComment(parts[0]);
   }
@@ skipping 605 matching lines @@
         .map((e) => originalMirror.getField(e.simpleName).reflectee)
         .where((e) => e != null)
         .toList();
   }
 
   Map toMap() => {
     'name': Indexable.getDocgenObject(mirror, owningLibrary).docName,
     'parameters': parameters
   };
 }