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)',

[ricow1, 2014/01/08 06:39:49]

shouldn't we depend on ../../sdk/bin/docgen{'',.bat} instead here (put it below
with dart2js)
By doing this we are depending on it indirectly through the create_sdk script, I
think it is better to have a direct input since it makes it easier to read this

[Alan Knight, 2014/01/08 22:19:02]

OK, but then shouldn't we also be running the one from that directory and not
from PRODUCT_DIR below? It seems like we could be in the position where the
PRODUCT_DIR one doesn't exist yet. But it doesn't work if I do that, so leaving
it (the one below) as is and changing this one.

             '<!@(["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',