Allow new FIDL syntax (#474)

Sets a flag on the fidlc compiler invocation that accepts the
new syntax, in preparation for converting all SDK FIDL files
to the new syntax.

Author: felixzhuologist

build/fuchsia/fidl_gen_cpp.py

@@ -54,7 +54,7 @@ def main():
   fidlc_command = [
     args.fidlc_bin,
     '--experimental',
-    'old_syntax_only',
+    'allow_new_syntax',
     '--tables',
     args.output_c_tables,
     '--json',