Do not throw when resolving a local engine if the .packages map does not...

Do not throw when resolving a local engine if the .packages map does not include the flutter package (#5519)

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -189,10 +189,12 @@ class FlutterCommandRunner extends CommandRunner {
     if (engineSourcePath == null && globalResults['local-engine'] != null) {
       try {
         Uri engineUri = new PackageMap(PackageMap.globalPackagesPath).map[kFlutterEnginePackageName];
-        engineSourcePath = path.dirname(path.dirname(path.dirname(path.dirname(engineUri.path))));
-        bool dirExists = FileSystemEntity.isDirectorySync(path.join(engineSourcePath, 'out'));
-        if (engineSourcePath == '/' || engineSourcePath.isEmpty || !dirExists)
-          engineSourcePath = null;
+        if (engineUri != null) {
+          engineSourcePath = path.dirname(path.dirname(path.dirname(path.dirname(engineUri.path))));
+          bool dirExists = FileSystemEntity.isDirectorySync(path.join(engineSourcePath, 'out'));
+          if (engineSourcePath == '/' || engineSourcePath.isEmpty || !dirExists)
+            engineSourcePath = null;
+        }
       } on FileSystemException { } on FormatException { }
 
       if (engineSourcePath == null)