Treat empty and uninitialized Maps the same in equality checks

[agreaves]

Treat an empty Map the same as an uninitialized Map when performing equality checks. This ensures that reading a map field does not alter the hashCode or equality comparisons of the message itself.

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[osa1]

Thanks for the PR. I think this does the right thing given the current representation of default values and uninitialized fields.

I will add a changelog entry for this.

Could you sign the CLA and format the code using dart format please?

We should also add a test since this fixes a bug. A simple repro:

syntax = "proto3";

message Test {
  map<int32, int32> my_map = 1;
}

import '../test.pb.dart';

void main() {
  Test test1 = Test.create();
  Test test2 = Test.create();

  assert(test1 == test2);
  assert(test1.hashCode == test2.hashCode);
  test1.myMap;
  assert(test1 == test2);
  assert(test1.hashCode == test2.hashCode);
}

Feel free to modify one of the existing tests instead. We have a bunch of messages with map fields in the tests that you may be able to reuse.

[osa1]

Closing in favor of #638.

[osa1]

Maintainer's note: internally an identical patch was implemented in cl/304375641.