Make daemon server work on ipv6-only machines.

[chingjun]

Retry binding on ipv6 if binding on ipv4 failed.

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[christopherfujino]

LGTM. can you add a test for this? I would:

1. Mark _DaemonServer visibleForTesting
2. Rather than directly calling ServerSocket.bind(), instead add a @visibleForTesting bind property on the class, that defaults to ServerSocket.bind()
3. Add a test to https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/general.shard/daemon_test.dart injecting a stateful bind callback that throw a SocketException if passed the ipv4 loopback, and then respond with a FakeServerSocket if passed the ipv6 loopback.
4. Have the test assert that we try first with the ipv4 loopback, then fall back to ipv6 after the exception.

[chingjun]

Done.

I've added the test in a new file test/general.shard/commands/daemon_test.dart instead, because test/general.shard/commands/daemon_test.dart is tests for lib/src/daemon.dart

PTAL, thanks!

[christopherfujino]

LGTM!