Enable and fix package:pedantic lints (#135)

- always_declare_return_types
- omit_local_variable_types
- prefer_collection_literals
- prefer_if_null_operators
- unawaited_futures

Bump min SDK to 2.2.0 to support Set literals.
diff --git a/example/example.dart b/example/example.dart
index 7bc2b4b..bb7b345 100644
--- a/example/example.dart
+++ b/example/example.dart
@@ -32,17 +32,17 @@
 }
 
 void _handleFoo() {
-  Analytics analytics = getAnalytics();
+  var analytics = getAnalytics();
   analytics.sendEvent('main', 'foo');
 }
 
 void _handleBar() {
-  Analytics analytics = getAnalytics();
+  var analytics = getAnalytics();
   analytics.sendEvent('main', 'bar');
 }
 
 void _changePage() {
-  Analytics analytics = getAnalytics();
+  var analytics = getAnalytics();
   window.history.pushState(null, 'new page', '${++_count}.html');
   analytics.sendScreenView(window.location.pathname);
 }