Ref https://github.com/tc39/test262/pull/1198#discussion_r137060514

Because Proxies no longer allow to return duplicate property keys.

Misc. class fields fixes

Incorrect $DONE handlers which led to calling $DONE twice
- async-generators/yield-promise-reject-next-yield-star-async-iterator.case
- dstr-assignment-for-await/array-elem-trlg-iter-rest-nrml-close-skip.case

$DONE handler not called at all:
- dstr-assignment-for-await/array-elem-put-const.case
- dstr-assignment-for-await/array-elem-trlg-iter-elision-iter-nrml-close-null.case

Incorrect assumed execution sequence in IteratorDestructuringAssignmentEvaluation:
- dstr-assignment-for-await/array-elem-iter-rtrn-close-null.case

- Correct property access in class fields template
- Remove assertions for properties which aren't present
- Correct expected assertion value for uninitialized class field
- Correct expected property descriptor attributes
- Fix property tests in class-fields/string-literal-names.case
- Pass restore option, otherwise it's not possible to call the method later
- Fix async iteration tests to test iteration result objects

Fix erroneous description for one of the `do ... while` tests

Typo in test/language/statements/labeled/continue.js

Basic tests for BigInt

Add tests for invalid BigInt MVs

Class fields

* Apply feedback for RegExp.prototype.flags

Ref #1149

Tests for NumericSeparatorLiteral. Closes gh-1051

Remove remaining cases for ...{

features: generator => generators. Fixes gh-1181

Removes previously missed object rest `...{` cases.