Don't spew DDC SDK build warnings when building.

Change-Id: I56b829e218a970ca8186910481422a5b9d25150c
Reviewed-on: https://dart-review.googlesource.com/22720
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>

tools/run_dart.py

@@ -28,6 +28,10 @@ def BuildArguments():
   result.add_argument("--dart",
                       help="dart executable",
                       default=None)
+  result.add_argument("-q", "--quiet",
+                      help="emit no output",
+                      default=False,
+                      action="store_true")
   return result
 
 def main():
@@ -43,8 +47,14 @@ def main():
     print >> sys.stderr, 'ERROR: cannot locate dart executable'
     return -1
 
-  subprocess.check_call([options.dart_executable] + args)
-  return 0
+  if options.quiet:
+    # Pipe output to /dev/null. See https://stackoverflow.com/a/14736249/9457.
+    out = open(os.devnull, 'w')
+  else:
+    out = None
+
+  return subprocess.call([options.dart_executable] + args,
+      stdout=out, stderr=out)
 
 if __name__ == '__main__':
   sys.exit(main())

utils/dartdevc/BUILD.gn

@@ -193,6 +193,7 @@ action("dartdevc_sdk") {
   }
 
   args = [
+    "--quiet",
     rebase_path("../../pkg/dev_compiler/tool/build_sdk.dart"),
     "--dart-sdk",
     rebase_path(patched_sdk_dir),