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

tools/observatory_tool.py

 #!/usr/bin/env python
 # Copyright (c) 2015, 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.
 """Helper for building and deploying Observatory"""
 
 import argparse
 import os
 import platform
 import shutil
@@ skipping 25 matching lines @@
   result.add_argument("--directory", help="observatory root", default=None)
   result.add_argument("--command", help="[get, build, deploy]", default=None)
   result.add_argument("--silent", help="silence all output", default=False)
   result.add_argument("--sdk", help="Use prebuilt sdk", default=False)
   return result
 
 def ProcessOptions(options, args):
   # Required options.
   if (options.command == None) or (options.directory == None):
     return False
-  # If we have a pub executable, we are running from the dart-sdk.
-  if (options.pub_executable != None):
-    return True
   if (options.sdk != None):
     # Use the checked in pub executable by default.
     options.pub_executable = utils.CheckedInPubPath()
-    return True
-  # Otherwise, we need a dart executable and a package root.
+  # If we have a working pub executable, we are running from the dart-sdk.
+  if (options.pub_executable != None):
+    try:
+      if utils.CheckedInSdkFixExecutable() and 0 == subprocess.call(
+             [options.pub_executable, '--version']):
+        return True
+    except OSError as e:
+      pass
+  # The pub executable didn't work, use the backup procedure.
+  options.pub_executable = None
+  # We need a dart executable and a package root.
   return ((options.package_root != None) and
           (options.dart_executable != None))
 
 def ChangeDirectory(directory):
   os.chdir(directory);
 
 def PubGet(dart_executable, pub_executable, pkg_root, silent):
   # Always remove pubspec.lock before running 'pub get'.
   try:
     os.remove('pubspec.lock');
@@ skipping 87 matching lines @@
     options.dart_executable = os.path.abspath(options.dart_executable)
   if (options.pub_executable != None):
     options.pub_executable = os.path.abspath(options.pub_executable)
   if len(args) == 1:
     args[0] = os.path.abspath(args[0])
   # Pub must be run from the project's root directory.
   ChangeDirectory(options.directory)
   return ExecuteCommand(options, args)
 
 if __name__ == '__main__':
-  sys.exit(main());
+  sys.exit(main());