[release] Allow promotion from one channel to another

The Travis `beta` channel can break when a new stable release is
published and packages use that release as minimum SDK constraint.
Therefore, we want to publish stable releases to beta if the current
beta is less than the current stable release. This change adds a
`--source-channel` option to promote.py that allows publishing a release
across channels.

Example (will promote the `2.8.1` stable release to beta):
```
$ python promote.py promote \
  --dry --channel=beta --source-channel=stable --version=2.8.1
```

Change-Id: Ieac4c33dfe17d4b38060e10ff3aff44494aae40a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146787
Reviewed-by: William Hesse <whesse@google.com>

tools/promote.py

@@ -46,7 +46,14 @@ def BuildOptions():
         help='The channel to promote.',
         default=None)
     group.add_option(
-        "--dry", help='Dry run', default=False, action="store_true")
+        '--source-channel',
+        type='string',
+        help='The channel to promote from. Defaults to the --channel value.',
+        default=None)
+    group.add_option('--dry',
+                     help='Dry run',
+                     default=False,
+                     action='store_true')
     result.add_option_group(group)
 
     return result
@@ -67,14 +74,16 @@ def main():
     if args[0] == 'promote':
         command = 'promote'
         if options.revision is None:
-            die('You must specify a --revision to specify which revision to promote'
-               )
+            die('You must specify the --version to promote')
 
         # Make sure options.channel is a valid
         if not options.channel:
             die('Specify --channel=beta/dev/stable')
         if options.channel not in bot_utils.Channel.ALL_CHANNELS:
-            die('You must supply a valid channel to --channel to promote')
+            die('You must supply a valid --channel to promote')
+        if (options.source_channel and
+                options.source_channel not in bot_utils.Channel.ALL_CHANNELS):
+            die('You must supply a valid --source-channel to promote from')
     else:
         die('Invalid command specified: {0}.  See help below'.format(args[0]))
 
@@ -82,13 +91,14 @@ def main():
         global DRY_RUN
         DRY_RUN = True
     if command == 'promote':
-        _PromoteDartArchiveBuild(options.channel, options.revision)
+        source = options.source_channel or options.channel
+        _PromoteDartArchiveBuild(options.channel, source, options.revision)
 
 
 def UpdateDocs():
     try:
         print 'Updating docs'
-        url = "http://api.dartlang.org/docs/releases/latest/?force_reload=true"
+        url = 'http://api.dartlang.org/docs/releases/latest/?force_reload=true'
         f = urllib.urlopen(url)
         f.read()
         print 'Successfully updated api docs'
@@ -97,11 +107,12 @@ def UpdateDocs():
         print 'Failed with: %s' % e
 
 
-def _PromoteDartArchiveBuild(channel, revision):
+def _PromoteDartArchiveBuild(channel, source_channel, revision):
     # These namer objects will be used to create GCS object URIs. For the
     # structure we use, please see tools/bots/bot_utils.py:GCSNamer
-    raw_namer = bot_utils.GCSNamer(channel, bot_utils.ReleaseType.RAW)
-    signed_namer = bot_utils.GCSNamer(channel, bot_utils.ReleaseType.SIGNED)
+    raw_namer = bot_utils.GCSNamer(source_channel, bot_utils.ReleaseType.RAW)
+    signed_namer = bot_utils.GCSNamer(source_channel,
+                                      bot_utils.ReleaseType.SIGNED)
     release_namer = bot_utils.GCSNamer(channel, bot_utils.ReleaseType.RELEASE)
 
     def promote(to_revision):
@@ -110,7 +121,7 @@ def _PromoteDartArchiveBuild(channel, revision):
             if not (revision != None and len(channel) > 0 and
                     ('%s' % revision) in gs_path and channel in gs_path):
                 raise Exception(
-                    "InternalError: Sanity check failed on GS URI: %s" %
+                    'InternalError: Sanity check failed on GS URI: %s' %
                     gs_path)
 
         def exists(gs_path):
@@ -176,7 +187,7 @@ def Gsutil(cmd, throw_on_error=True):
     gsutilTool = join(DART_PATH, 'third_party', 'gsutil', 'gsutil')
     command = [sys.executable, gsutilTool] + cmd
     if DRY_RUN:
-        print "DRY runnning: %s" % command
+        print 'DRY runnning: %s' % command
         return (None, None, 0)
     return bot_utils.run(command, throw_on_error=throw_on_error)