Convert Flask.run into a noop when run from the CLI

flask/cli.py

@@ -412,6 +412,13 @@ def run_command(info, host, port, reload, debugger, eager_loading,
     """
     from werkzeug.serving import run_simple
 
+    # Set a global flag that indicates that we were invoked from the
+    # command line interface provided server command.  This is detected
+    # by Flask.run to make the call into a no-op.  This is necessary to
+    # avoid ugly errors when the script that is loaded here also attempts
+    # to start a server.
+    os.environ['FLASK_RUN_FROM_CLI_SERVER'] = '1'
+
     debug = get_debug_flag()
     if reload is None:
         reload = bool(debug)