`web/charts` is broken on deploy

[Brett Morgan]

From https://github.com/flutter/samples/runs/3810622434#step:5:303:

charts (3 of 11)
  Running `flutter pub get`
Running "flutter pub get" in charts...                           2,377ms
  Success! – charts – `flutter pub get`
  Running `flutter build web`

Building without sound null safety
For more information see https://dart.dev/null-safety/unsound-null-safety

Compiling lib/main.dart for the Web...                          
Target dart2js failed: Exception: /opt/hostedtoolcache/flutter/2.5.2-stable/x64/.pub-cache/hosted/pub.dartlang.org/charts_flutter-0.10.0/lib/src/behaviors/legend/legend_entry_layout.dart:134:45:
Error: The getter 'body1' isn't defined for the class 'TextTheme'.
 - 'TextTheme' is from 'package:flutter/src/material/text_theme.dart' ('/opt/hostedtoolcache/flutter/2.5.2-stable/x64/packages/flutter/lib/src/material/text_theme.dart').
      color ??= Theme.of(context).textTheme.body1.color;
                                            ^^^^^
Error: Compilation failed.

Compiling lib/main.dart for the Web...                             11.7s
Exception: Failed to compile application for the Web.
  Failed! (1) – charts – `flutter build web`