run pub get (w/ symlinks) for flutter drive (#4259)

* run pub get (w/ symlinks) for flutter drive

* review comments

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

@@ -12,8 +12,11 @@ import '../android/android_device.dart' show AndroidDevice;
 import '../application_package.dart';
 import '../base/file_system.dart';
 import '../base/common.dart';
+import '../base/logger.dart';
 import '../base/os.dart';
+import '../base/process.dart';
 import '../build_info.dart';
+import '../dart/sdk.dart';
 import '../device.dart';
 import '../globals.dart';
 import '../ios/simulators.dart' show SimControl, IOSSimulatorUtils;
@@ -121,6 +124,16 @@ class DriveCommand extends RunCommandBase {
       printStatus('Will connect to already running application instance.');
     }
 
+    // Check for the existance of a `packages/` directory; pub test does not yet
+    // support running without symlinks.
+    if (!new io.Directory('packages').existsSync()) {
+      Status status = logger.startProgress(
+        'Missing packages directory; running `pub get` (to work around https://github.com/dart-lang/test/issues/327):'
+      );
+      await runAsync(<String>[sdkBinaryName('pub'), 'get', '--no-precompile']);
+      status.stop(showElapsedTime: true);
+    }
+
     try {
       return await testRunner(<String>[testFile])
         .catchError((dynamic error, dynamic stackTrace) {