Add download of the stable SDK from gcloud to the gclient hooks in DEPS.

tools/sdks/README

Index: tools/sdks/README
diff --git a/tools/sdks/README b/tools/sdks/README
new file mode 100644
index 0000000000000000000000000000000000000000..f1f1fd356723154bda0caf3cceb9ec8a193e258f
--- /dev/null
+++ b/tools/sdks/README
@@ -0,0 +1,20 @@
+This directory contains the hashes of tar.gz files uploaded to cloud storage.
+Hooks in the DEPS files run "download_from_google_storage" to download these
+tarfiles, and unpack them.  These tar files contain a stable Dart SDK, which
+is used to build the observatory and to run the test scripts.
+
+The hooks use the --auto-platform feature, so that only the SDK for the
+current platform (linux, macos, or windows) is downloaded.  This requires
+the subdirectories to have the special names "linux", "win", and "mac", which
+the download script is hardcoded to recognize.
+
+The linux SDK has had two extra dart executables added to the bin directory.
+'dart-mips' and 'dart-arm' are executables compiled to run on mips and arm
+processors, respectively.
+
+To upload new versions of these tar files, use the "upload_to_google_storage"
+tool in depot_tools, and download the new stable SDKs from the dartlang.org
+web page.  The mips and arm executables must be copied from the machines that
+build them, and stripped.  There should be no need to update these tar files
+for every new stable version of Dart.
+