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

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 26 matching lines @@
 /// simple server to view the documentation.
 ///
 /// Returned Future completes with true if document generation is successful.
 Future<bool> docgen(List<String> files, {String packageRoot,
     bool includePrivate: false, bool includeSdk: false, bool parseSdk: false,
     String introFileName: '', String out: gen.DEFAULT_OUTPUT_DIRECTORY,
     List<String> excludeLibraries: const [],
     bool includeDependentPackages: false, bool compile: false,
     bool serve: false, bool noDocs: false, String startPage,
     String pubScript : 'pub', String dartBinary: 'dart',
-    bool indentJSON: false}) {
+    bool indentJSON: false, String sdk}) {
   var result;
   if (!noDocs) {
     viewer.ensureMovedViewerCode();
     result = gen.generateDocumentation(files, packageRoot: packageRoot,
         includePrivate: includePrivate,
         includeSdk: includeSdk, parseSdk: parseSdk,
         introFileName: introFileName, out: out,
         excludeLibraries: excludeLibraries,
         includeDependentPackages: includeDependentPackages,
         startPage: startPage, pubScriptValue: pubScript,
-        dartBinaryValue: dartBinary, indentJSON: indentJSON);
+        dartBinaryValue: dartBinary, indentJSON: indentJSON, sdk: sdk);
     viewer.addBackViewerCode();
     if (compile || serve) {
       result.then((success) {
         if (success) {
           viewer.createViewer(serve);
         }
       });
     }
   } else if (compile || serve) {
     gen.pubScript = pubScript;
     gen.dartBinary = dartBinary;
     viewer.createViewer(serve);
   }
   return result;
 }