Wait for sky server to start before starting device

Instead of just waiting for the sky server process to start
before we start the activity on the device, this causes us to
wait for the sky server to actually start listening on its port

Fixes #141

packages/flutter_tools/lib/src/device.dart

@@ -5,6 +5,7 @@
 library sky_tools.device;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 
@@ -512,6 +513,9 @@ class AndroidDevice extends Device {
   static const String className = 'AndroidDevice';
   static final String defaultDeviceID = 'default_android_device';
 
+  static const String _kFlutterServerStartMessage = 'Serving';
+  static const Duration _kFlutterServerTimeout = const Duration(seconds: 3);
+
   String productID;
   String modelID;
   String deviceCodeName;
@@ -773,8 +777,14 @@ class AndroidDevice extends Device {
           [adbPath, 'forward', observatoryPortString, observatoryPortString]);
 
       // Actually start the server.
-      await Process.start(sdkBinaryName('pub'), ['run', 'sky_tools:sky_server', _serverPort],
-          workingDirectory: serverRoot, mode: ProcessStartMode.DETACHED);
+      Process server = await Process.start(
+          sdkBinaryName('pub'), ['run', 'sky_tools:sky_server', _serverPort],
+          workingDirectory: serverRoot,
+          mode: ProcessStartMode.DETACHED_WITH_STDIO
+      );
+      await server.stdout.transform(UTF8.decoder)
+          .firstWhere((String value) => value.startsWith(_kFlutterServerStartMessage))
+          .timeout(_kFlutterServerTimeout);
 
       // Set up reverse port-forwarding so that the Android app can reach the
       // server running on localhost.