fix(http2): reject requests that exceed SETTINGS_MAX_CONCURRENT_STREAMS

[brianquinlan]

1. Return REFUSED_STREAM if the peer requests more peers then were advertised in SETTINGS_MAX_CONCURRENT_STREAMS. This is a requirement according to RFC-7540 5.1.2.
2. Modify the tests/format/etc. to run with the stable version of Dart - the dev and stable versions of Dart currently disagree on formatting.

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