Generate `bytes` with `Uint8List`

Ref: https://groups.google.com/a/dartlang.org/forum/#!topic/misc/JpNkRRLI9_w, https://github.com/Daegalus/dart-uuid/issues/35

More and more packages are explicitly switching to Uint8List

Right now, with the folling proto
```proto
message BluetoothCharacteristic {
bytes value = 6;
}
```

The generated dart code (by `Activated protoc_plugin 16.0.1.`) is
```dart
import 'dart:core' show int, bool, double, String, List, Map, override;

import 'package:protobuf/protobuf.dart' as $pb;

class BluetoothCharacteristic extends $pb.GeneratedMessage {
static final $pb.BuilderInfo _i = new $pb.BuilderInfo('BluetoothCharacteristic')
..a>(6, 'value', $pb.PbFieldType.OY)
..hasRequiredFields = false
;

BluetoothCharacteristic() : super();
BluetoothCharacteristic.fromBuffer(List i, [$pb.ExtensionRegistry r = $pb.ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
BluetoothCharacteristic.fromJson(String i, [$pb.ExtensionRegistry r = $pb.ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
BluetoothCharacteristic clone() => new BluetoothCharacteristic()..mergeFromMessage(this);
BluetoothCharacteristic copyWith(void Function(BluetoothCharacteristic) updates) => super.copyWith((message) => updates(message as BluetoothCharacteristic));
$pb.BuilderInfo get info_ => _i;
static BluetoothCharacteristic create() => new BluetoothCharacteristic();
BluetoothCharacteristic createEmptyInstance() => create();
static $pb.PbList createRepeated() => new $pb.PbList();
static BluetoothCharacteristic getDefault() => _defaultInstance ??= create()..freeze();
static BluetoothCharacteristic _defaultInstance;

List get value => $_getN(0);
set value(List v) { $_setBytes(0, v); }
bool hasValue() => $_has(0);
void clearValue() => clearField(6);
}
```

We can see that we still have
```dart
List get value => $_getN(0);
set value(List v) { $_setBytes(0, v); }
```

Expected generated code:
```dart
Uint8List get value => $_getN(0);
set value(Uint8List v) { $_setBytes(0, v); }
```

One concern might be that this is more or less a breaking change in strong mode.

Start in the protoc_plugin code path that emits Dart types for protobuf bytes fields, using the BluetoothCharacteristic example as the reference case. Check existing generated-code coverage for bytes fields and determine how Uint8List affects strong-mode compatibility. Done means generated bytes accessors use Uint8List with appropriate coverage and the compatibility concern is addressed.

Written by the indexing model from the issue text.