[ddc] Only print blank line in ddb if verbose.

The blank line clobbers the output if ddb is used in non-verbose mode
to execute a program where the stdout ordinarily belongs to the program
in question whose output might be machine parsed.

Change-Id: Ie929d19deabebb7b1f3d24f99035aec2d9844a64
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153208
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>

pkg/dev_compiler/tool/ddb

@@ -227,7 +227,9 @@ void main(List<String> args) async {
   }
 
   // Print an initial empty line to separate the invocation from the output.
-  print('');
+  if (verbose) {
+    print('');
+  }
 
   if (compile) {
     var ddcArgs = [