Update TextTheme.button.letterSpacing from 0.75 to 1.25 per spec (#59008)

packages/flutter/lib/src/material/typography.dart

@@ -526,7 +526,7 @@ class Typography with Diagnosticable {
     bodyText2 : TextStyle(debugLabel: 'englishLike bodyText2 2018', fontSize: 14.0, fontWeight: FontWeight.w400, textBaseline: TextBaseline.alphabetic, letterSpacing: 0.25),
     subtitle1 : TextStyle(debugLabel: 'englishLike subtitle1 2018', fontSize: 16.0, fontWeight: FontWeight.w400, textBaseline: TextBaseline.alphabetic, letterSpacing: 0.15),
     subtitle2 : TextStyle(debugLabel: 'englishLike subtitle2 2018', fontSize: 14.0, fontWeight: FontWeight.w500, textBaseline: TextBaseline.alphabetic, letterSpacing: 0.1),
-    button    : TextStyle(debugLabel: 'englishLike button 2018',    fontSize: 14.0, fontWeight: FontWeight.w500, textBaseline: TextBaseline.alphabetic, letterSpacing: 0.75),
+    button    : TextStyle(debugLabel: 'englishLike button 2018',    fontSize: 14.0, fontWeight: FontWeight.w500, textBaseline: TextBaseline.alphabetic, letterSpacing: 1.25),
     caption   : TextStyle(debugLabel: 'englishLike caption 2018',   fontSize: 12.0, fontWeight: FontWeight.w400, textBaseline: TextBaseline.alphabetic, letterSpacing: 0.4),
     overline  : TextStyle(debugLabel: 'englishLike overline 2018',  fontSize: 10.0, fontWeight: FontWeight.w400, textBaseline: TextBaseline.alphabetic, letterSpacing: 1.5),
   );

packages/flutter/test/material/typography_test.dart

@@ -167,8 +167,7 @@ void main() {
     expect(theme.button.fontFamily, 'Roboto');
     expect(theme.button.fontWeight, medium);
     expect(theme.button.fontSize, 14);
-    // TODO(hansmuller): restore after englishLike2018 has been updated, per #38904.
-    //expect(theme.button.letterSpacing, 1.25);
+    expect(theme.button.letterSpacing, 1.25);
 
     // Caption Roboto regular 12 0.4
     expect(theme.caption.fontFamily, 'Roboto');