Don't use reserved words as identifier

This bug took me some time to figure out. I got very strage errors in combination with ```pub build``` that I couldn't figure out first:

```
ERR : Error on line 286 of protobuf|lib/generated_message.dart: Expected to find
 ')'
    |       enums.forEach((ProtobufEnum enum) {
    |                                   ^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Expected to find
 ')'
    |       enums.forEach((ProtobufEnum enum) {
    |                                   ^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |       enums.forEach((ProtobufEnum enum) {
    |                      ^^^^^^^^^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Expected a state
ment
    |       enums.forEach((ProtobufEnum enum) {
    |                                   ^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Unexpected token
 'enum'
    |       enums.forEach((ProtobufEnum enum) {
    |                                   ^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |       enums.forEach((ProtobufEnum enum) {
    |                                       ^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |       enums.forEach((ProtobufEnum enum) {
    |                                   ^^^^
    |
    | Error on line 286 of protobuf|lib/generated_message.dart: Unexpected token
 ')'
    |       enums.forEach((ProtobufEnum enum) {
    |                                       ^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                             ^^^^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                           ^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected a state
ment
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                             ^^^^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                 ^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                  ^^^^^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                       ^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                  ^^^^^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Unexpected token
 ')'
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                       ^
    |
    | Error on line 287 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |         hash = (31 * hash + enum.value) & 0x3fffffff;
    |                                         ^
    |
    | Error on line 288 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |       });
    |        ^
    |
    | Error on line 288 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |       });
    |       ^
    |
    | Error on line 288 of protobuf|lib/generated_message.dart: Unexpected token
 ')'
    |       });
    |        ^
    |
    | Error on line 303 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |           ProtobufEnum enum = value;
    |           ^^^^^^^^^^^^
    |
    | Error on line 303 of protobuf|lib/generated_message.dart: Expected a state
ment
    |           ProtobufEnum enum = value;
    |                        ^^^^
    |
    | Error on line 303 of protobuf|lib/generated_message.dart: Unexpected token
 'enum'
    |           ProtobufEnum enum = value;
    |                        ^^^^
    |
    | Error on line 303 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |           ProtobufEnum enum = value;
    |                             ^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                 ^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected to find
 ')'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                 ^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                               ^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected a state
ment
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                 ^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Unexpected token
 'enum'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                 ^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                     ^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                      ^^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                           ^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected to find
 ';'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                      ^^^^^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Unexpected token
 ')'
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                           ^
    |
    | Error on line 304 of protobuf|lib/generated_message.dart: Expected an iden
tifier
    |           hash = ((53 * hash) + enum.value) & 0x3fffffff;
    |                                             ^
    |

```

But this morning I found [this](https://groups.google.com/a/dartlang.org/forum/#!topic/misc/AbSe5N_x1bc) post on the Dart Misc mailing list. That post got me on the right track. I took a look into the language spec of Dart. [Looks like](https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.huusvrzea3q) ```enum``` is a reserved word in Dart and can not be used as an identifier (enum isn't used yet, but probably reserved for future use).

This pull request only renames the identifer from ```enum``` to ```protobufEnum``` to solve the problem.