Prepare for making `intl` `toBeginningOfSentenceCase` non-nullable (#127488)

I intend to edit `toBeginningOfSentenceCase`'s return value to be non-nullable because it really is never null. That will mean that non-null asserts around it will become flagged as unnecessary, although right now they are necessary. So, apply a workaround - instead use a function that does a non-null assert without triggering any lints even after it becomes unnecessary.

packages/flutter_tools/lib/src/base/utils.dart

@@ -59,7 +59,8 @@ String sentenceCase(String str, [String? locale]) {
   if (str.isEmpty) {
     return str;
   }
-  return toBeginningOfSentenceCase(str, locale)!;
+  // TODO(christopherfujino): Remove this check after the next release of intl
+  return ArgumentError.checkNotNull(toBeginningOfSentenceCase(str, locale));
 }
 
 /// Converts `foo_bar` to `Foo Bar`.