Add a snapshot for docgen and use it in the build

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 24 matching lines @@
 import '../../../sdk/lib/_internal/libraries.dart';
 
 var logger = new Logger('Docgen');
 
 const DEFAULT_OUTPUT_DIRECTORY = 'docs';
 
 var _outputDirectory;
 
 const String USAGE = 'Usage: dart docgen.dart [OPTIONS] fooDir/barFile';
 
-
 List<String> skippedAnnotations = const [
     'metadata.DocsEditable', '_js_helper.JSName', '_js_helper.Creates',
     '_js_helper.Returns'];
 
+/**

[Bob Nystrom, 2014/01/08 00:16:57]

Use a ///-style doc comment like the rest of this file.

[Alan Knight, 2014/01/08 01:22:52]

Done.

+ * If we can't find the SDK introduction text, which will happen if running
+ * from a snapshot and using --parse-sdk or --include-sdk, then use this
+ * hard-coded version. This should be updated to be consistent with the text
+ * in docgen/doc/sdk-introduction.md
+ */
+const String DEFAULT_SDK_INTRODUCTION = """

[Bob Nystrom, 2014/01/08 00:16:57]

You don't need to type annotate constants.

[Alan Knight, 2014/01/08 01:22:52]

Done.

+Welcome to the Dart API reference documentation,
+covering the official Dart API libraries.
+Some of the most fundamental Dart libraries include:
+
+* [dart:core](#dart:core):
+  Core functionality such as strings, numbers, collections, errors,
+  dates, and URIs.
+* [dart:html](#dart:html):
+  DOM manipulation for web apps.
+* [dart:io](#dart:io):
+  I/O for command-line apps.
+
+Except for dart:core, you must import a library before you can use it.
+Here's an example of importing dart:html, dart:math, and a
+third popular library called
+[polymer.dart](http://www.dartlang.org/polymer-dart/):
+
+    import 'dart:html';
+    import 'dart:math';
+    import 'package:polymer/polymer.dart';
+
+Polymer.dart is an example of a library that isn't
+included in the Dart download,
+but is easy to get and update using the _pub package manager_.
+For information on finding, using, and publishing libraries (and more)
+with pub, see
+[pub.dartlang.org](http://pub.dartlang.org).
+
+The main site for learning and using Dart is
+[www.dartlang.org](http://www.dartlang.org).
+Check out these pages:
+
+  * [Dart homepage](http://www.dartlang.org)
+  * [Tutorials](http://www.dartlang.org/docs/tutorials/)
+  * [Programmer's Guide](http://www.dartlang.org/docs/)
+  * [Samples](http://www.dartlang.org/samples/)
+  * [A Tour of the Dart Libraries](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
+
+This API reference is automatically generated from the source code in the
+[Dart project](https://code.google.com/p/dart/).
+If you'd like to contribute to this documentation, see
+[Contributing](https://code.google.com/p/dart/wiki/Contributing)
+and
+[Writing API Documentation](https://code.google.com/p/dart/wiki/WritingApiDocumentation).
+""";
+
 /// Set of libraries declared in the SDK, so libraries that can be accessed
 /// when running dart by default.
 Iterable<LibraryMirror> _sdkLibraries;
 
 /// The dart:core library, which contains all types that are always available
 /// without import.
 LibraryMirror _coreLibrary;
 
 /// Support for [:foo:]-style code comments to the markdown parser.
 List<markdown.InlineSyntax> markdownSyntaxes =
@@ skipping 290 matching lines @@
   // Everything is a subclass of Object, therefore empty the list to avoid a
   // giant list of subclasses to be printed out.
   if (includeSdk) (entityMap['dart-core.Object'] as Class).subclasses.clear();
 
   var filteredEntities = entityMap.values.where(_isVisible);
 
   // Outputs a JSON file with all libraries and their preview comments.
   // This will help the viewer know what libraries are available to read in.
   var libraryMap;
   var linkResolver = (name) => fixReference(name, null, null, null);
+
+  String readIntro(String introFileName) {
+    var introFile = new File(introFileName);
+    var introText = introFile.existsSync() ? introFile.readAsStringSync :
+        DEFAULT_SDK_INTRODUCTION;
+    return markdown.markdownToHtml(introText,
+        linkResolver: linkResolver, inlineSyntaxes: markdownSyntaxes);
+  }
+
   if (append) {
     var docsDir = listDir(_outputDirectory);
     if (!docsDir.contains('$_outputDirectory/library_list.json')) {
       throw new StateError('No library_list.json');
     }
     libraryMap =
         JSON.decode(new File(
             '$_outputDirectory/library_list.json').readAsStringSync());
     libraryMap['libraries'].addAll(filteredEntities
         .where((e) => e is Library)
         .map((e) => e.previewMap));
     if (introduction.isNotEmpty) {
       var intro = libraryMap['introduction'];
       if (intro.isNotEmpty) intro += '<br/><br/>';
-      intro += markdown.markdownToHtml(
-          new File(introduction).readAsStringSync(),
-              linkResolver: linkResolver, inlineSyntaxes: markdownSyntaxes);
-      libraryMap['introduction'] = intro;
+      libraryMap['introduction'] = "$intro${readIntro(introduction)}";
     }
     outputToYaml = libraryMap['filetype'] == 'yaml';
   } else {
     libraryMap = {
       'libraries' : filteredEntities.where((e) =>
           e is Library).map((e) => e.previewMap).toList(),
-      'introduction' : introduction == '' ?
-          '' : markdown.markdownToHtml(new File(introduction)
-              .readAsStringSync(), linkResolver: linkResolver,
-                  inlineSyntaxes: markdownSyntaxes),
+      'introduction' : introduction.isEmpty ? '' : readIntro(introduction),
       'filetype' : outputToYaml ? 'yaml' : 'json'
     };
   }
   _writeToFile(JSON.encode(libraryMap), 'library_list.json');
 
   // Output libraries and classes to file after all information is generated.
   filteredEntities.where((e) => e is Class || e is Library).forEach((output) {
     _writeIndexableToFile(output, outputToYaml);
   });
 
@@ skipping 1303 matching lines @@
 /// Remove statics from the map of inherited items before adding them.
 Map _filterStatics(Map items) {
   var result = {};
   items.forEach((name, item) {
     if (!item.isStatic) {
       result[name] = item;
     }
   });
   return result;
 }