Add a snapshot for docgen and use it in the build

utils/compiler/compiler.gyp

Index: utils/compiler/compiler.gyp
diff --git a/utils/compiler/compiler.gyp b/utils/compiler/compiler.gyp
index 294c7e904325543666c311cb4403fa6898ee0a2f..12d068665b4cc2463181e52482188230a711f1df 100644
--- a/utils/compiler/compiler.gyp
+++ b/utils/compiler/compiler.gyp
@@ -20,7 +20,7 @@
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '../../sdk/lib/_internal/libraries.dart',
-            '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../sdk/lib/_internal/compiler", "../../runtime/lib", "../../sdk/lib/_internal/dartdoc"])',
+            '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../sdk/lib/_internal/compiler", "../../runtime/lib", "../../sdk/lib/_internal/dartdoc", "../../pkg/docgen"])',
             'create_snapshot.dart',
             '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
             '../../tools/VERSION',
@@ -35,6 +35,7 @@
             '--output_dir=<(SHARED_INTERMEDIATE_DIR)',
             '--dart2js_main=sdk/lib/_internal/compiler/implementation/dart2js.dart',
             '--dartdoc_main=sdk/lib/_internal/dartdoc/bin/dartdoc.dart',
+            '--docgen_main=pkg/docgen/bin/docgen.dart',
             '--package_root=<(PRODUCT_DIR)/packages/',
           ],
         },