Assertion failure when building test protos with assertions enabled

[osa1]

Just realized while debugging #605 with assertions that in master branch we currently have assertion failures when test protos are being compiled with assertions enabled. To reproduce enable assertions with this patch:

diff --git a/protoc_plugin/bin/protoc-gen-dart b/protoc_plugin/bin/protoc-gen-dart
index b1f8169..86b62aa 100755
--- a/protoc_plugin/bin/protoc-gen-dart
+++ b/protoc_plugin/bin/protoc-gen-dart
@@ -1,3 +1,3 @@
 #!/bin/bash
 BINDIR=$(dirname "$0")
-dart "$BINDIR/protoc_plugin.dart" -c "$@"
+dart --enable-asserts "$BINDIR/protoc_plugin.dart" -c "$@"

Now build test protos as usual:

$ make protos
[ -d out/protos ] || mkdir -p out/protos
protoc\
		--experimental_allow_proto3_optional\
		--dart_out=out/protos\
		-Iprotos\
		-Itest/protos\
		--plugin=protoc-gen-dart=/Users/omersa/dart/protobuf.dart/protoc_plugin/bin/protoc-gen-dart\
		 test/protos/_leading_underscores.proto  test/protos/google/protobuf/any.proto  test/protos/google/protobuf/api.proto  test/protos/google/protobuf/duration.proto  test/protos/google/protobuf/empty.proto  test/protos/google/protobuf/field_mask.proto  test/protos/google/protobuf/source_context.proto  test/protos/google/protobuf/struct.proto  test/protos/google/protobuf/timestamp.proto  test/protos/google/protobuf/type.proto  test/protos/google/protobuf/unittest_import.proto  test/protos/google/protobuf/unittest_optimize_for.proto  test/protos/google/protobuf/unittest_well_known_types.proto  test/protos/google/protobuf/unittest.proto  test/protos/google/protobuf/wrappers.proto  test/protos/custom_option.proto  test/protos/dart_name.proto  test/protos/default_value_escape.proto  test/protos/enum_extension.proto  test/protos/enum_name.proto  test/protos/extend_unittest.proto  test/protos/ExtensionEnumNameConflict.proto  test/protos/ExtensionNameConflict.proto  test/protos/foo.proto  test/protos/high_tagnumber.proto  test/protos/import_clash.proto  test/protos/import_public.proto  test/protos/json_name.proto  test/protos/map_api.proto  test/protos/map_api2.proto  test/protos/map_enum_value.proto  test/protos/map_field.proto  test/protos/mixins.proto  test/protos/multiple_files_test.proto  test/protos/nested_extension.proto  test/protos/non_nested_extension.proto  test/protos/oneof.proto  test/protos/reserved_names.proto  test/protos/reserved_names_extension.proto  test/protos/reserved_names_message.proto  test/protos/duplicate_names_import.proto  test/protos/package1.proto  test/protos/package2.proto  test/protos/package3.proto  test/protos/proto3_optional.proto  test/protos/service.proto  test/protos/service2.proto  test/protos/service3.proto  test/protos/toplevel_import.proto  test/protos/toplevel.proto  test/protos/using_any.proto
Unhandled exception:
'package:protoc_plugin/src/protobuf_field.dart': Failed assertion: line 313 pos 12: '!isRepeated': is not true.
#0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:51:61)
#1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:40:5)
#2      ProtobufField.generateDefaultFunction (package:protoc_plugin/src/protobuf_field.dart:313:12)
#3      ProtobufField.generateBuilderInfoCall (package:protoc_plugin/src/protobuf_field.dart:225:39)
#4      MessageGenerator.generate.<anonymous closure>.<anonymous closure> (package:protoc_plugin/src/message_generator.dart:345:29)
#5      IndentingWriter._addBlockBodyAndEnd (package:protoc_plugin/indenting_writer.dart:95:9)
#6      IndentingWriter.addBlock (package:protoc_plugin/indenting_writer.dart:73:5)
#7      MessageGenerator.generate.<anonymous closure> (package:protoc_plugin/src/message_generator.dart:331:11)
#8      IndentingWriter._addBlockBodyAndEnd (package:protoc_plugin/indenting_writer.dart:95:9)
#9      IndentingWriter.addAnnotatedBlock (package:protoc_plugin/indenting_writer.dart:88:5)
#10     MessageGenerator.generate (package:protoc_plugin/src/message_generator.dart:310:9)
#11     FileGenerator.generateMainFile (package:protoc_plugin/src/file_generator.dart:273:9)
#12     FileGenerator.generateFiles (package:protoc_plugin/src/file_generator.dart:232:22)
#13     CodeGenerator.generate.<anonymous closure> (package:protoc_plugin/src/code_generator.dart:127:36)
<asynchronous suspension>
--dart_out: protoc-gen-dart: Plugin failed with status code 255.
make: *** [out/protos/_leading_underscores.pb.dart] Error 1

In general we should run the tests with the assertions enabled.