JS Async functions and await #104

Implement async functions and the await expression.

Scope#

Async Functions#

• async function declarations and expressions
• async arrow functions: async () => { ... }
• async method definitions in objects and classes
• Async functions always return a Promise
• Return value wraps in Promise.resolve()
• Thrown exception wraps in Promise.reject()

Await Expression#

• await expr — pause async function until promise settles
• If awaited value is fulfilled, resume with fulfillment value
• If awaited value is rejected, throw rejection reason
• Awaiting non-promise values: wrap in Promise.resolve first
• Proper microtask scheduling (await is a microtask boundary)

Async Iteration#

• async function* (async generator functions)
• for await...of loop
• Async iterators: { next() → Promise<{ value, done }> }
• Symbol.asyncIterator

Integration#

• Async functions build on the Promise and generator infrastructure
• The VM must support suspending and resuming execution across await points
• Microtask queue integration: await resumes via microtask

Acceptance Criteria#

• Async functions return Promises
• await pauses execution until promise settles
• await with fulfilled promise resumes with value
• await with rejected promise throws
• Multiple awaits in sequence work correctly
• Async arrow functions work
• try/catch in async functions catches rejected awaits
• Async generators yield promises
• for await...of consumes async iterables
• Proper microtask ordering with await
• Unit tests for all async patterns

Phase 10 — JavaScript Engine (issue 15a of 15). Depends on: JS Promise, JS iterators/generators.