Enable always_declare_return_types lint

And fix a zillion omissions this uncovered.

packages/flutter_tools/bin/flutter_tools.dart

diff --git a/packages/flutter_tools/bin/flutter_tools.dart b/packages/flutter_tools/bin/flutter_tools.dart
index 482239c..762e6a5 100644
--- a/packages/flutter_tools/bin/flutter_tools.dart
+++ b/packages/flutter_tools/bin/flutter_tools.dart
@@ -4,4 +4,6 @@
 
 import 'package:flutter_tools/executable.dart' as executable;
 
-main(List<String> args) => executable.main(args);
+void main(List<String> args) {
+  executable.main(args);
+}