Observatory build should try backup method if the SDK doesn't work.

tools/utils.py

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This file contains a set of utilities functions used by other Python-based
 # scripts.
 
 import commands
 import datetime
 import json
@@ skipping 547 matching lines @@
   print 'Executing: ' + ' '.join(cmd)
   pipe = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
       shell=IsWindows())
   output = pipe.communicate()
   if pipe.returncode != 0:
     raise Exception('Execution failed: ' + str(output))
   return pipe.returncode, output
 
 
 def DartBinary():
+  # TODO(24311): Replace all uses of this with CheckedInSdk[Fix]Executable().
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   dart_binary_prefix = os.path.join(tools_dir, 'testing', 'bin')
   if IsWindows():
     return os.path.join(dart_binary_prefix, 'windows', 'dart.exe')
   else:
     arch = GuessArchitecture()
     system = GuessOS()
     if arch == 'armv5te':
       # TODO(zra): This binary does not exist, yet. Check one in once we have
       # sufficient stability.
       return os.path.join(dart_binary_prefix, system, 'dart-armv5te')
     elif arch == 'arm':
       return os.path.join(dart_binary_prefix, system, 'dart-arm')
     elif arch == 'arm64':
       return os.path.join(dart_binary_prefix, system, 'dart-arm64')
     elif arch == 'mips':
       return os.path.join(dart_binary_prefix, system, 'dart-mips')
     else:
       return os.path.join(dart_binary_prefix, system, 'dart')
 
 
 def DartSdkBinary():
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   dart_binary_prefix = os.path.join(tools_dir, '..', 'sdk' , 'bin')
   return os.path.join(dart_binary_prefix, 'dart')
 
 
+# The checked-in SDKs are documented at
+#     https://github.com/dart-lang/sdk/wiki/The-checked-in-SDK-in-tools
 def CheckedInSdkPath():
   # We don't use the normal macos, linux, win32 directory names here, instead,
   # we use the names that the download_from_google_storage script uses.
   osdict = {'Darwin':'mac', 'Linux':'linux', 'Windows':'win'}
   system = platform.system()
   try:
     osname = osdict[system]
   except KeyError:
     print >>sys.stderr, ('WARNING: platform "%s" not supported') % (system)
     return None;
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   return os.path.join(tools_dir,
                       'sdks',
                       osname,
                       'dart-sdk')
 
 
-def CheckedInPubPath():
-  executable_name = 'pub'
-  if platform.system() == 'Windows':
-    executable_name = 'pub.bat'
-  return os.path.join(CheckedInSdkPath(), 'bin', executable_name)
+def CheckedInSdkExecutable():
+  name = 'dart'
+  if IsWindows():
+    name = 'dart.exe'
+  elif GuessOS() == 'linux':
+    arch = GuessArchitecture()
+    if arch == 'mips':
+      name = 'dart-mips'
+    elif arch == 'arm':
+      name = 'dart-arm'
+  return os.path.join(CheckedInSdkPath(), 'bin', name)
+
+
+def CheckedInSdkCheckExecutable():
+  executable = CheckedInSdkExecutable()
+  canary_script = os.path.join(os.path.dirname(os.path.realpath(__file__)),
+                               'canary.dart')
+  try:
+    if 42 == subprocess.call([executable, canary_script]):
+      return True
+  except OSError as e:
+    pass
+  return False
 
 
 class TempDir(object):
   def __init__(self, prefix=''):
     self._temp_dir = None
     self._prefix = prefix
 
   def __enter__(self):
     self._temp_dir = tempfile.mkdtemp(self._prefix)
     return self._temp_dir
@@ skipping 11 matching lines @@
     os.chdir(self._working_directory)
 
   def __exit__(self, *_):
     print "Enter directory = ", self._old_cwd
     os.chdir(self._old_cwd)
 
 
 if __name__ == "__main__":
   import sys
   Main()