Closure conversion: support async

[karlklose]

Currently, async support is done partly by kernel and partly in the VM, which relies on a specific set of variables and use of first-class functions, which is not easy for closure conversion to deal with.

[karlklose]

Most of the language tests (127/175) failing with closure conversion end in a stacktrace like this:

  [0x00000000009aa673] dart::Profiler::DumpStackTrace()
  [0x00000000009aa673] dart::Profiler::DumpStackTrace()
  [0x000000000068fc11] dart::DynamicAssertionHelper::Fail(char const*, ...)
  [0x0000000000980f4e] dart::kernel::ScopeBuilder::AddExceptionVariable(dart::GrowableArray<dart::LocalVariable*>*, char const*, long)
  [0x000000000098389d] dart::kernel::ScopeBuilder::VisitTryCatch(dart::kernel::TryCatch*)
  [0x0000000000a91e90] dart::kernel::Block::VisitChildren(dart::kernel::Visitor*)
  [0x0000000000983119] non-virtual thunk to dart::kernel::ScopeBuilder::VisitBlock(dart::kernel::Block*)
  [0x0000000000983d86] dart::kernel::ScopeBuilder::VisitFunctionNode(dart::kernel::FunctionNode*)
  [0x0000000000982438] dart::kernel::ScopeBuilder::BuildScopes()
  [0x0000000000925615] dart::ParsedFunction::EnsureKernelScopes()
  [0x000000000098f3d3] dart::kernel::FlowGraphBuilder::BuildGraph()
  [0x00000000006f2d42] dart::DartCompilationPipeline::BuildFlowGraph(dart::Zone*, dart::ParsedFunction*, dart::ZoneGrowableArray<dart::ICData const*> const&, long)
  [0x00000000006f5c38] dart::CompileParsedFunctionHelper::Compile(dart::CompilationPipeline*)
  [0x00000000006f86c3] Unknown symbol
  [0x00000000006f7fee] dart::Compiler::CompileFunction(dart::Thread*, dart::Function const&)
  [0x00000000006f30fb] dart::DRT_CompileFunction(dart::NativeArguments)
  [0x00007ffff7e43630] [Stub] CallToRuntime
  [0x00007ffff7e436b0] [Stub] LazyCompile
  [0x00007ffff62882e9] Closure#Future#microtask#function.call
  [0x00007ffff6287ee5] _microtaskLoop
  [0x00007ffff628799c] _startMicrotaskLoop
  [0x00007ffff6287852] Closure#_startMicrotaskLoop.call
  [0x00007ffff6287732] _runPendingImmediateCallback
  [0x00007ffff628129f] _RawReceivePortImpl._handleMessage
  [0x00007ffff7e43a4e] [Stub] InvokeDartCode
  [0x0000000000710cd8] dart::DartEntry::InvokeFunction(dart::Function const&, dart::Array const&, dart::Array const&, unsigned long)
  [0x0000000000710a74] dart::DartEntry::InvokeFunction(dart::Function const&, dart::Array const&)
  [0x0000000000713e44] dart::DartLibraryCalls::HandleMessage(dart::Object const&, dart::Instance const&)
  [0x000000000082091d] dart::IsolateMessageHandler::HandleMessage(dart::Message*)
  [0x0000000000855919] dart::MessageHandler::HandleMessages(dart::MonitorLocker*, bool, bool)
  [0x0000000000855fd2] dart::MessageHandler::TaskCallback()
  [0x0000000000856a43] Unknown symbol