Download the dart sdk into third_party/.

This is meant to be a temporary change. We call wget and unzip from a
python script. In a few weeks, this script should be replaced with
a call to download_from_google_storage once that script understands
archives.

BUG=459376
R=zra@google.com

Review URL: https://codereview.chromium.org/932993003
diff --git a/DEPS b/DEPS
index a12af1d..bb15498 100644
--- a/DEPS
+++ b/DEPS
@@ -235,6 +235,12 @@
     'action': ['python', 'src/tools/clang/scripts/update.py', '--if-needed'],
   },
   {
+    # Pull dart sdk if needed
+    'name': 'dart',
+    'pattern': '.',
+    'action': ['python', 'src/tools/dart/update.py'],
+  },
+  {
     # Update the Windows toolchain if necessary.
     'name': 'win_toolchain',
     'pattern': '.',