Merge pull request #227 from yjbanov/sync-v0.2

Consolidate mixologist generator code
consistent quotes
cleanup equality utilities
add tracing events to ProxyClient and JSON parser
optional marshallers and API resources; generate RawEntity; port tests
is_map mixin
fixes in http handler code
make error handling in SimpleRequestHandler behave like ProxyClient does
close streams on errors; nested microtasks do not affect onDone
freeze entity in ProxyClient
honor request headers even without a payload, and add withCredentials support
track response/error objects in relevant trace events
tests for DotAccess
reformat code in emitter.dart