[Bug] Cannot default export an expression that begins with async

[Seikho]

TypeScript Version: nightly (2.0.0-dev.20160705)

The changes in #9445 have caused a breaking change in my code base. This is a pattern that we use very frequently our projects.
The compiled output has also changed and no longer functions as expected. The same error also occurs when using the async library in the same way (e.g. export default async.map(...))

Code

import {async,await} from 'asyncawait';
export default async(() => await(Promise.resolve(1)));

should compile without error and emit:

const asyncawait_1 = require('asyncawait');
Object.defineProperty(exports, "__esModule", { value: true });
exports.default = asyncawait_1.async(() => asyncawait_1.await(Promise.resolve(1)));

but incorrectly emits:

const asyncawait_1 = require('asyncawait');
asyncawait_1.async(() => asyncawait_1.await(Promise.resolve(1)));

Expected behavior:
Compiles without error
Actual behavior:
[ts] Declaration expected
Incorrect compiled output

[yortus]

Looking at the changes in #9445, it seems that if the next token after export default is async, then it's treated like a declaration rather than an expression. But async is not a keyword (spec), and a valid expression may begin with an async token, as in @Seikho's two examples above.

I think export default <expression> should be allowed even if the expression is something like async.map(...) or async(() => {...}), since these are valid expressions.

[yortus]

If this makes it into the 2.0 release, it will be a breaking change.

[DanielRosenwasser]

Thanks for tracking the regression down @yortus!