SnapshotCreator: start from existing snapshot if we have one

tools/dev/gm.py

 #!/usr/bin/env python
 # Copyright 2017 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """\
 Convenience wrapper for compiling V8 with gn/ninja and running tests.
 Sets up build output directories if they don't exist.
 Produces simulator builds for non-Intel target architectures.
 Uses Goma by default if it is detected (at output directory setup time).
 Expects to be run from the root of a V8 checkout.
@@ skipping 270 matching lines @@
     for c in self.configs:
       self.configs[c].Extend(self.global_targets, self.global_tests)
     return self.configs
 
 def Main(argv):
   parser = ArgumentParser()
   configs = parser.ParseArguments(argv[1:])
   return_code = 0
   for c in configs:
     return_code += configs[c].Build()
-  for c in configs:
-    return_code += configs[c].RunTests()
+  if return_code == 0:
+    for c in configs:
+      return_code += configs[c].RunTests()
   if return_code == 0:
     _Call("notify-send 'Done!' 'V8 compilation finished successfully.'",
           silent=True)
   else:
     _Call("notify-send 'Error!' 'V8 compilation finished with errors.'",
           silent=True)
   return return_code
 
 if __name__ == "__main__":
   sys.exit(Main(sys.argv))