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

tools/observatory_tool.py

Index: tools/observatory_tool.py
diff --git a/tools/observatory_tool.py b/tools/observatory_tool.py
index adf103fde62b31c32ee4ab930f40256ae9c9de62..ffc76df24ab34fc72c46c26c5c967e3b8d411d07 100755
--- a/tools/observatory_tool.py
+++ b/tools/observatory_tool.py
@@ -43,14 +43,20 @@ 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))
 
@@ -158,4 +164,4 @@ def main():
   return ExecuteCommand(options, args)
 
 if __name__ == '__main__':
-  sys.exit(main());
+  sys.exit(main());