Add a snapshot for docgen and use it in the build

utils/compiler/create_snapshot.dart

Index: utils/compiler/create_snapshot.dart
diff --git a/utils/compiler/create_snapshot.dart b/utils/compiler/create_snapshot.dart
index b7bb18b74109a98bec9402391673c5163e2ef722..96c6fcb527ae905b3b42828dba0e95a7024c1015 100644
--- a/utils/compiler/create_snapshot.dart
+++ b/utils/compiler/create_snapshot.dart
@@ -19,11 +19,13 @@ Future<String> getVersion(var rootPath) {
 Future<String> getSnapshotGenerationFile(var args, var rootPath) {
   var dart2js = rootPath.resolve(args["dart2js_main"]);
   var dartdoc = rootPath.resolve(args["dartdoc_main"]);
+  var docgen = rootPath.resolve(args["docgen_main"]);
   return getVersion(rootPath).then((version) {
     var snapshotGenerationText =
 """
 import '${dart2js.toFilePath(windows: false)}' as dart2jsMain;
 import '${dartdoc.toFilePath(windows: false)}' as dartdocMain;
+import '${docgen.toFilePath(windows: false)}' as docgenMain;
 import 'dart:io';
 
 void main(List<String> arguments) {
@@ -34,6 +36,8 @@ void main(List<String> arguments) {
     dart2jsMain.main(arguments.skip(1).toList());
   } else if (tool == "dartdoc") {
     dartdocMain.main(arguments.skip(1).toList());
+  } else if (tool == "docgen") {
+    docgenMain.main(arguments.skip(1).toList());
   }
 }
 
@@ -84,7 +88,7 @@ Future createSnapshot(var dart_file, var packageRoot) {
  */
 void main(List<String> arguments) {
   var validArguments = ["--output_dir", "--dart2js_main", "--dartdoc_main",
-                        "--package_root"];
+                        "--docgen_main", "--package_root"];
   var args = {};
   for (var argument in arguments) {
     var argumentSplit = argument.split("=");
@@ -96,6 +100,7 @@ void main(List<String> arguments) {
   }
   if (!args.containsKey("dart2js_main")) throw "Please specify dart2js_main";
   if (!args.containsKey("dartdoc_main")) throw "Please specify dartdoc_main";
+  if (!args.containsKey("docgen_main")) throw "Please specify docgen_main";
   if (!args.containsKey("output_dir")) throw "Please specify output_dir";
   if (!args.containsKey("package_root")) throw "Please specify package_root";