Update pubspecs (#121)

Add compatibility up to Dart 3.0, to fix building with Flutter head,
which now uses 2.1 (with nothing specified, <2 is assumed as the
requirement).

Also removes the example's pubspeck.lock and adds it to the ignore
list until the project has a plan in place for specifying the required
version of Flutter.

Author: stuartmorgan

example_flutter/.gitignore

@@ -9,3 +9,8 @@ build/
 ios/
 packages
 .flutter-plugins
+# Since the current expectation is that clients are using Flutter head,
+# and Dart SDK changes unlock dependencies, pubspec.lock isn't useful.
+# Ignore it since it just creates noise. When there's a system in place
+# for versioning this project relative to Flutter, remove this.
+pubspec.lock