Fixes dev SDK download script to sniff for the version Flutter wants

tools/download_latest_dev_sdk.py

Index: tools/download_latest_dev_sdk.py
diff --git a/tools/download_latest_dev_sdk.py b/tools/download_latest_dev_sdk.py
index c5de4202594eb48a830b103663b6751900f1cf29..d39cae4ee5353109b25ad3d6cf5120aebe94462f 100755
--- a/tools/download_latest_dev_sdk.py
+++ b/tools/download_latest_dev_sdk.py
@@ -18,9 +18,11 @@ HOST_OS = utils.GuessOS()
 HOST_ARCH = utils.GuessArchitecture()
 SCRIPT_DIR = os.path.dirname(sys.argv[0])
 DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..'))
+FUCHSIA_ROOT = os.path.realpath(os.path.join(DART_ROOT, '..'))
+FLUTTER_ROOT = os.path.join(FUCHSIA_ROOT, 'lib', 'flutter')
 
-DART_VERSION = '1.25.0-dev.7.0'
-BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/' + DART_VERSION +'/sdk'
+DEFAULT_DART_VERSION = 'latest'
+BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/%s/sdk/%s'
 
 def host_os_for_sdk(host_os):
   if host_os.startswith('macos'):
@@ -45,8 +47,18 @@ def main(argv):
   local_sha_path = os.path.join(sdk_path, sha_file)
   remote_sha_path = os.path.join(sdk_path, sha_file + '.remote')
   zip_path = os.path.join(sdk_path, zip_file)
-  sha_url = BASE_URL + '/' + sha_file
-  zip_url = BASE_URL + '/' + zip_file
+
+  # If we're in a Fuchsia checkout with Flutter nearby, pull the same dev SDK
+  # version that Flutter says it wants. Otherwise, pull the latest dev SDK.
+  sdk_version_path = os.path.join(
+      FLUTTER_ROOT, 'bin', 'internal', 'dart-sdk.version')
+  sdk_version = DEFAULT_DART_VERSION
+  if os.path.isfile(sdk_version_path):
+    with open(sdk_version_path, 'r') as fp:
+      sdk_version = fp.read().strip()
+
+  sha_url = (BASE_URL % (sdk_version, sha_file))
+  zip_url = (BASE_URL % (sdk_version, zip_file))
 
   local_sha = ''
   if os.path.isfile(local_sha_path):