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

tools/build.py

 #!/usr/bin/env python
 #
 # 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.
 #
 
 import optparse
 import os
 import re
@@ skipping 59 matching lines @@
       metavar=HOST_CPUS,
       default=str(HOST_CPUS))
   (vs_directory, vs_executable) = utils.GuessVisualStudioPath()
   result.add_option("--devenv",
       help='Path containing devenv.com on Windows',
       default=vs_directory)
   result.add_option("--executable",
       help='Name of the devenv.com/msbuild executable on Windows (varies for '
            'different versions of Visual Studio)',
       default=vs_executable)
-  result.add_option("--use-bootstrap-for-observatory",
-      help='Use a stripped down Dart binary built on the host machine '
-           'for building Observatory. Necessary on Linux machines which have '
-           'libc incompatibilities with the prebuilt Dart binaries.',
-      default=False, action="store_true")
   return result
 
 
 def ProcessOsOption(os_name):
   if os_name == 'host':
     return HOST_OS
   return os_name
 
 
 def ProcessOptions(options, args):
@@ skipping 289 matching lines @@
   if command:
     # Ignore return code, if this command fails, it doesn't matter.
     os.system(command)
 
 
 filter_xcodebuild_output = False
 def BuildOneConfig(options, target, target_os, mode, arch, override_tools):
   global filter_xcodebuild_output
   start_time = time.time()
   os.environ['DART_BUILD_MODE'] = mode
-  if options.use_bootstrap_for_observatory != False:
-    os.environ['DART_USE_BOOTSTRAP_BIN'] = '1'
   build_config = utils.GetBuildConf(mode, arch, target_os)
   if HOST_OS == 'macos':
     filter_xcodebuild_output = True
     project_file = 'dart.xcodeproj'
     if os.path.exists('dart-%s.gyp' % CurrentDirectoryBaseName()):
       project_file = 'dart-%s.xcodeproj' % CurrentDirectoryBaseName()
     args = ['xcodebuild',
             '-project',
             project_file,
             '-target',
@@ skipping 151 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())