Implement const/let pattern declarations (ES6 destructuring)

[dsgkirkby]

This should resolve #415.

Details

Relevant part of the spec here: https://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations-runtime-semantics-evaluation

For // 3. Let env be the running execution context’s LexicalEnvironment., I assumed that the env variable passed to the function already fulfills and it's not necessary to retrieve it here.

I've omitted the ReturnIfAbrupt(Value) steps from the spec, as this is also done for the rest of the implementation in this file.

Testing

test262 has one additional test pass as a result of this change, though somewhat surprisingly it's for for.
I've verified in the command line REPL that destructuring assignment works as expected in the basic case.

[NTillmann]

Looks great!

Please fix the Flow error...
In scripts/test262-runner.js, please update this line to reflect increased test passes:

if (!args.filterString && (numPassedES5 < 11738 || numPassedES6 < 3981 || numTimeouts > 0)) {

Also, take a closer look at that file... There are various pattern matching heuristics in place to filter out entire chunks of the test262 test suite that didn't use to work, including lots of destructuring tests. Maybe some of those can be enabled now?

[dsgkirkby]

I tried removing the filter for destructuring-binding (results for ES6 tests only):

All tests

Master: 5427 / 6116 (689 failed - 92.9% pass rate)
Branch: 5684 / 6116 (432 failed - 91.6% pass rate)

Added tests only

Master: 1168 / 1468 (300 failed - 79.6% pass rate)
Branch: 1389 / 1468 (79 failed - 97.1% pass rate)

I haven't dug into exactly what's failing but I think that's probably good enough to enable these tests?

[dsgkirkby]

Update:

• It seems that circleci runs less tests than locally so I've lowered the expected passes to match how many pass there
• The added failures are due to lexical environments being leaked, which looks like it might be an issue with generators - those failures are happening on master for some generator-related tests

[NTillmann]

Awesome! Thanks a lot for doing this.

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