Implement Array.prototype.flat(Map) with some basic tests

[mstoykov]

No description provided.

[mstoykov]

I will be making some more as part of our try to drop corejs completely.

The list so far is :

• Array.prototype.flat(Map) - this PR
• String.prototype.trimLeft/Right
• String.prototype.matchAll
• Object.prototype.getOwnPropertyDescriptors
• Object.prototype.values/entries

Possibly (although I hope not):

• Object.prototype.defineSetter/defineGetter and the lookup part

[dop251]

Thanks for submitting. I have added a couple of minor changes, otherwise looks good to me.
The only thing is most array methods have optimisations for simple arrays (see checkStdArray) where property access can be replaced with operating directly on a.values. It's not necessary though and could be added at a later stage should there be a need...

[mstoykov]

After the latest changes the only flat* test that are failing are :

--- FAIL: TestTC39 (4.22s)
    --- FAIL: TestTC39/test262 (4.22s)
        --- FAIL: TestTC39/test262/test/built-ins/Array/prototype/flat/null-undefined-elements.js (0.00s)
        Error Trace:    tc39_test.go:219
                        tc39_test.go:233
                        tc39_test.go:280
                        tc39_test.go:392
                        tc39_test.go:515
                        tc39_norace_test.go:11
        Error:          Should be empty, but was [test/built-ins/Array/prototype/flat/null-undefined-elements.js TypeError: Cannot convert undefined or null to object at flat (native)]: %!v(MISSING)
        Test:           TestTC39/test262/test/built-ins/Array/prototype/flat/null-undefined-elements.js
        --- FAIL: TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-bad-throws.js (0.45s)
        Error Trace:    tc39_test.go:219
                        tc39_test.go:233
                        tc39_test.go:280
                        tc39_test.go:392
                        tc39_test.go:515
                        tc39_norace_test.go:11
        Error:          Should be empty, but was [test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-bad-throws.js Test262Error: throw TypeError if @@species is a number Expected a TypeError to be thrown but no exception was thrown at all at harness/sta.js:22:9(49)]: %!v(MISSING)
        Test:           TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-bad-throws.js
        --- FAIL: TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor-poisoned-throws.js (0.17s)
        Error Trace:    tc39_test.go:219
                        tc39_test.go:233
                        tc39_test.go:280
                        tc39_test.go:392
                        tc39_test.go:515
                        tc39_norace_test.go:11
        Error:          Should be empty, but was [test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor-poisoned-throws.js Test262Error: Return abrupt completion from species custom ctor Expected a Test262Error to be thrown but no exception was thrown at all at harness/sta.js:22:9(49)]: %!v(MISSING)
        Test:           TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor-poisoned-throws.js
        --- FAIL: TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor.js (0.20s)
        Error Trace:    tc39_test.go:219
                        tc39_test.go:233
                        tc39_test.go:280
                        tc39_test.go:392
                        tc39_test.go:515
                        tc39_norace_test.go:11
        Error:          Should be empty, but was [test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor.js Test262Error: returned value is an instance of custom ctor at harness/sta.js:22:9(49)]: %!v(MISSING)
        Test:           TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species-custom-ctor.js
        --- FAIL: TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species.js (0.53s)
        Error Trace:    tc39_test.go:219
                        tc39_test.go:233
                        tc39_test.go:280
                        tc39_test.go:392
                        tc39_test.go:515
                        tc39_norace_test.go:11
        Error:          Should be empty, but was [test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species.js Test262Error: got species once Expected SameValue(«0», «1») to be true at harness/sta.js:22:9(49)]: %!v(MISSING)
        Test:           TestTC39/test262/test/built-ins/Array/prototype/flatMap/this-value-ctor-object-species.js
FAIL
exit status 1

For the record this was run through k6 which also employs babel to get some additional ES support including template variables.

I don't think I can do much about any of those?

but maybe I should now write the fast path? Including moving the check for the map function outside the loop and having 2 loops?