Make coverage.py optional for dm and nanobench _flags.py

tools/nanobench_flags.py

 #
 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 
 #!/usr/bin/env python
 
 usage = '''
@@ skipping 126 matching lines @@
 
   if match:
     args.append('--match')
     args.extend(match)
 
   return args
 cov_end = lineno()   # Don't care about code coverage past here.
 
 
 def self_test():
-  import coverage  # This way the bots don't need coverage.py to be installed.
   args = {}
   cases = [
     'Perf-Android-GCC-Nexus6-GPU-Adreno420-Arm7-Release',
     'Perf-Android-Nexus7-Tegra3-Arm7-Release',
     'Perf-Android-GCC-NexusPlayer-GPU-PowerVR-x86-Release',
     'Perf-Android-GCC-GalaxyS3-GPU-Mali400-Arm7-Release',
     'Test-Ubuntu-GCC-ShuttleA-GPU-GTX550Ti-x86_64-Release-Valgrind',
     'Test-Win7-MSVC-ShuttleA-GPU-HD2000-x86-Debug-ANGLE',
     'Test-iOS-Clang-iPad4-GPU-SGX554-Arm7-Debug',
     'Test-Android-GCC-GalaxyS4-GPU-SGX544-Arm7-Release',
     'Perf-Android-GCC-NVIDIA_Shield-GPU-TegraX1-Arm64-Release',
     'Perf-Android-GCC-NVIDIA_Shield-GPU-TegraX1-Arm64-Release-Vulkan',
     'Perf-Android-GCC-Nexus5-GPU-Adreno330-Arm7-Release',
   ]
 
-  cov = coverage.coverage()
-  cov.start()
-  for case in cases:
-    args[case] = get_args(case)
-  cov.stop()
+  this_file = os.path.basename(__file__)
+  try:
+    import coverage
+    cov = coverage.coverage()
+    cov.start()
+    for case in cases:
+      args[case] = get_args(case)
+    cov.stop()
 
-  this_file = os.path.basename(__file__)
-  _, _, not_run, _ = cov.analysis(this_file)
-  filtered = [line for line in not_run if line > cov_start and line < cov_end]
-  if filtered:
-    print 'Lines not covered by test cases: ', filtered
-    sys.exit(1)
+    _, _, not_run, _ = cov.analysis(this_file)
+    filtered = [line for line in not_run if line > cov_start and line < cov_end]
+    if filtered:
+      print 'Lines not covered by test cases: ', filtered
+      sys.exit(1)
+  except ImportError:
+    print ("We cannot guarantee that this files tests are comprehensive " +
+           "without coverage.py.  Please install it when you get a chance.")
 
   golden = this_file.replace('.py', '.json')
   with open(os.path.join(os.path.dirname(__file__), golden), 'w') as f:
     json.dump(args, f, indent=2, sort_keys=True)
 
 
 if __name__ == '__main__':
   if len(sys.argv) == 2 and sys.argv[1] == 'test':
     self_test()
     sys.exit(0)
 
   if len(sys.argv) != 3:
     print usage
     sys.exit(1)
 
   with open(sys.argv[1], 'w') as out:
     json.dump(get_args(sys.argv[2]), out)