This patch traces all public async methods and wraps them
in a helper method that tags the sync stack trace.
Later on, if the method call throws an exception, we add
a captured stack trace to the original stack trace with the "--ASYNC--"
heading.
An example of a stack trace:
```
Error: net::ERR_ABORTED at http://localhost:8907/empty.html
at navigate (/Users/lushnikov/prog/puppeteer/lib/Page.js:622:37)
at process._tickCallback (internal/process/next_tick.js:68:7)
-- ASYNC --
at Page.<anonymous> (/Users/lushnikov/prog/puppeteer/lib/helper.js:147:27)
at fit (/Users/lushnikov/prog/puppeteer/test/page.spec.js:546:18)
at process._tickCallback (internal/process/next_tick.js:68:7)
```
0b9d8a6271