Update `FieldInfo._ensureRepeatedField` return type #1204

Job	Run time
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	34s
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	34s
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	36s
mono_repo self validate	26s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	25s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	38s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	58s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	39s
run_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	1m 9s
run_tests; linux; Dart dev; PKG: protobuf; `dart test`	35s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	1m 6s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	47s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	1m 14s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	55s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	50s
	11m 26s

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