Bug fix: Correctly return value for foo = await bar in console

[haltman-at]

Addresses #3349.

This PR fixes the issue where expressions of the form foo = await bar, when entered in the Truffle Console, would perform the assignment but then not return the resulting value. Note that this PR does not fix the more general issues with the use of await in the Truffle Console, which is issue #1546. Fixing the broader problems are probably going to have to wait until we have removed support for both Node 12 and Node 14.

The fundamental problem, and the reason for #1546, is that all our await-handling code in the console is a bunch of hacks. The ultimate solution to all of this, once we don't need to support Node 12 and Node 14 anymore, will be to remove all these hacks and do things the straightforward way that Node 16 will allow us to do it. However, in the meantime, I still wanted to fix the particular issue of #3349. To accomplish that, this PR uses... more hacks. But, it's not worse than the already existing hacks surrounding it, so I think it's fine.

The reason that we get undefined in this case is because we have special handling for this particular case, and that special handling involves creating a little assignment script and then passing it to a VM to run; we do get the return value from our script, but the problem is that we wrote the script in a way such that it always returns undefined. So, I've modified it to (conditionally) return the assigned value instead.

Specifically, if we are doing a bare assignment (so no let or const, those shouldn't return values), I modify the assignment script by appending the variable name to the end of it, so that the value is returned. This solves the problem! Or the narrow problem of #3349, anyway. The broad problem of #1546 will have to wait...

(Although, you can do things beyond just foo = await bar with it... e.g., x = (await web3.eth.net.getId()) + 1 will work fine!)

I didn't include any tests in this PR because um I'm not sure how to test this programmatically. But potentially tests could be added if people think it's necessary (and if anyone can give me an idea for how, or point me to where we already do this sort of thing...)

[cds-amal]

Nice one @haltman-at! I'll be able to review this tomorrow. This can be tested by utilizing the same setup for console/develop REPL tests.

    assert.equal(
      output.indexOf('undefined'),
      -1,
      "should not print `undefined` on naked assignments"
    );

[haltman-at]

Oh, I see! I only looked at console, not develop, so I didn't see that. Cool, I can go back and add a test then! (Of course I won't be abusing indexOf like that, sheesh, we have better ways these days. :P )

[cds-amal]

Nice one Harry. It's a good approach given the constraints we have. Of course, it would be nice to get a few tests in.

[haltman-at]

OK, added the test!

[cds-amal]

Looks good! Nice work @haltman-at