Undeclared variables being assigned when used with preset env targeting ES5

[i-like-robots]

When using the babel-plugin-transform-async-to-promises along with @babel/preset-env targeting an ES5 environment (such as IE 11) variables can be assigned without first being declared which causes the following error/s:

ReferenceError: assignment to undeclared variable varname

I have created a minimum test case demonstrating this issue here: https://github.com/i-like-robots/broken-babel-async-await-transform-test-case

There appears to be several ways to mitigate the issue as manifested in the example above:

• Change the target to an ES6 compatible environment so that the variable declarations to not need transforming to use var syntax (not an option for us)
• Move the export default statement to another line (most practical but in our case would require changes to number of code bases across our projects)
• Avoid using certain variables within the scope of a function generated by this plugin (probably not possible)

This may be a duplicate of #38

[rpetrich]

Babel's scope tracking got more strict in babel 7.6 and broke the tracking of variables across functions; this is now remedied in master and will be in the next release.

[i-like-robots]

Thank you @rpetrich 👍