Update TextTheme.button.letterSpacing from 0.75 to 1.25 per spec

[HansMuller]

This is the last change needed to bring Flutter's Typography in line with the 2018 Material spec.

Fixes #38904

This change has a small impact on buttons' default width. The top button's letterSpacing has the new default value (1.25), the lower button has the original default value (0.75).

It's easy enough to restore the original letterSpacing value for the TextTheme's button TextStyle by configuring the app's Theme.

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

void main() {
  final ThemeData theme = ThemeData.from(colorScheme: ColorScheme.light());
  runApp(
    MaterialApp(
      theme: theme.copyWith(
        typography: Typography.material2018(platform: defaultTargetPlatform),
        // The following override reverts the default letterSpacing change
        textTheme: theme.textTheme.copyWith(
          button: theme.textTheme.button.copyWith(letterSpacing: 0.75),
        ),
      ),
      home: Home(),
    )
  );
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print(Theme.of(context).textTheme.button.letterSpacing);
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            // Rendered with the default 2018 button letterSpacing change.
            RaisedButton(
              color: Theme.of(context).colorScheme.primary,
              textColor: Theme.of(context).colorScheme.onPrimary,
              onPressed: () { },
              child: Text(
                'BUTTON',
              ),
            ),
            // Rendered as it would be without the letterSpacing change.
            RaisedButton(
              onPressed: () { },
              color: Theme.of(context).colorScheme.primary,
              child: Text(
                'BUTTON',
                style: Theme.of(context).textTheme.button.copyWith(
                  letterSpacing: 0.75,
                  color: Theme.of(context).colorScheme.onPrimary,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

[darrenaustin]

LGTM.