Add a snapshot for docgen and use it in the build

utils/apidoc/docgen.gyp

Index: utils/apidoc/docgen.gyp
diff --git a/utils/apidoc/docgen.gyp b/utils/apidoc/docgen.gyp
index 27cc69d1feb500e1a21ff7b6a45fcf25d41dfdd4..0dc555c5c9d5ba883bd817b879a88c4ed44f0b22 100644
--- a/utils/apidoc/docgen.gyp
+++ b/utils/apidoc/docgen.gyp
@@ -57,6 +57,7 @@
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '<(SHARED_INTERMEDIATE_DIR)/utils_wrapper.dart.snapshot',
+            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)/dart-sdk/bin/docgen<(script_suffix)',
             '<!@(["python", "../../tools/list_files.py", "\\.(css|ico|js|json|png|sh|txt|yaml|py)$", ".", "../../sdk/lib/_internal/dartdoc"])',
             '<!@(["python", "../../tools/list_files.py", "\\.dart$", ".", "../../sdk/lib", "../../runtime/lib", "../../runtime/bin"])',
             '../../sdk/bin/dart',
@@ -76,10 +77,7 @@
             '../../tools/only_in_release_mode.py',
             '<@(_outputs)',
             '--',
-            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
-            '--old_gen_heap_size=1024',
-            '--package-root=<(PRODUCT_DIR)/packages/',
-            '../../pkg/docgen/bin/docgen.dart',
+            '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart-sdk/bin/docgen<(script_suffix)',
             '--out=<(PRODUCT_DIR)/api_docs/docgen',
             '--json',
             '--include-sdk',