Fix overly specific detection of non-UTF8 files in analyzer bot. (#56295)

An upcoming Dart SDK change
(https://github.com/dart-lang/sdk/commit/fa2fd41166db35afa4777e63f900e83d25709c5c)
changes the precise text of the exception generated by `utf8.decode`
if a non-UTF8 file is found.  This is causing a breakage in the Dart
team's `flutter-analyze` bot (and will presumably cause a breakage in
the corresponding Flutter bot as soon as this change is rolled into
Flutter).  To avoid this breakage, the bot shouldn't rely on the exact
exception text; it is sufficient to simply catch a FormatException.

dev/bots/analyze.dart

@@ -1086,13 +1086,9 @@ Future<void> verifyNoBinaries(String workingDirectory, { Set<Hash256> grandfathe
       try {
         utf8.decode(bytes);
       } on FormatException catch (error) {
-        if (error.message.startsWith('Bad UTF-8 encoding ')) {
-          final Digest digest = sha256.convert(bytes);
-          if (!grandfatheredBinaries.contains(Hash256.fromDigest(digest)))
-            problems.add('${file.path}:${error.offset}: file is not valid UTF-8');
-        } else {
-          rethrow;
-        }
+        final Digest digest = sha256.convert(bytes);
+        if (!grandfatheredBinaries.contains(Hash256.fromDigest(digest)))
+          problems.add('${file.path}:${error.offset}: file is not valid UTF-8');
       }
     }
     if (problems.isNotEmpty) {