Have shelldb and skydb automatically download assets for you

sky/tools/skydb

Index: sky/tools/skydb
diff --git a/sky/tools/skydb b/sky/tools/skydb
index 22194295cc00c602c98575f7fdfb3e467c385600..1159013334e78a9d15aa140d896454b2de3e9360 100755
--- a/sky/tools/skydb
+++ b/sky/tools/skydb
@@ -58,6 +58,16 @@ def gn_args_from_build_dir(build_dir):
     return config
 
 
+def ensure_assets_are_downloaded(build_dir):
+    sky_pkg_dir = os.path.join(build_dir, 'gen', 'dart-pkg', 'sky')
+    sky_pkg_lib_dir = os.path.join(sky_pkg_dir, 'lib')
+    sky_icons_dir = \
+        os.path.join(sky_pkg_lib_dir, 'assets', 'material-design-icons')
+    if not os.path.isdir(sky_icons_dir):
+        print('NOTE: Running `download_material_design_icons` for you.')
+        subprocess.check_call(
+            [os.path.join(sky_pkg_lib_dir, 'download_material_design_icons')])
+
 class SkyDebugger(object):
     def __init__(self):
         self.pids = {}
@@ -180,6 +190,8 @@ class SkyDebugger(object):
         gn_args = gn_args_from_build_dir(self.paths.build_dir)
         is_android = 'android_sdk_version' in gn_args
 
+        ensure_assets_are_downloaded(args.build_dir)
+
         shell_found = True
         if is_android:
             apk_path = os.path.join(self.paths.build_dir, 'apks', ANDROID_APK_NAME)