create build directory for startup trace if missing (#5589)

packages/flutter_tools/lib/src/commands/trace.dart

@@ -155,9 +155,14 @@ class Tracing {
 Future<Null> downloadStartupTrace(VMService observatory) async {
   File traceInfoFile = new File('build/start_up_info.json');
 
+  // Delete old startup data, if any.
   if (await traceInfoFile.exists())
     await traceInfoFile.delete();
 
+  // Create "build" directory, if missing.
+  if (!(await traceInfoFile.parent.exists()))
+    await traceInfoFile.parent.create();
+
   Tracing tracing = new Tracing(observatory);
 
   Map<String, dynamic> timeline = await tracing.stopTracingAndDownloadTimeline(