Skip for-in properties shadowed by non-enumerable properties - #2557

Fixes #1394.

for-in visited a prototype's enumerable property even when an object earlier in the chain had a non-enumerable property of the same name:

Object.prototype.length = 42;
for (var k in Function) print(k); // printed "length"

enumChangeObject added only the ids from getIds(), the enumerable ones, to the set of ids to skip. Objects without enumerable properties, such as Function and Function.prototype here, were passed over without adding anything. In EnumerateObjectProperties, a non-enumerable property of an object visited earlier hides a prototype's property of the same name.

The enumeration now keeps the ScriptableObjects it has left. Before it enumerates the next object that has enumerable properties, it adds their non-enumerable ids (getAllIds() minus getIds()) to that set.

• Why it waits: a for-in over a large array, whose prototypes have no enumerable properties, never collects the array's indexes.
• Only non-enumerable ids: enumerable properties added to an object during the loop must still not hide the prototype's, as before.
• Proxies are skipped: asking again would call the ownKeys trap twice. With a trap, getIds() already returns every key it lists. Without one, a proxy behaves as before.
• Other Scriptables: they behave as before.
• Typed arrays: their getIds() lists only indexes, so their expando properties now count as non-enumerable here and hide same-named prototype properties. for-in already skips those expandos.

Tests:

• six cases in ForTest:

  • the issue's example;
  • a direct shadow;
  • a shadowing object with no enumerable properties;
  • an array's length;
  • a property added during the loop;
  • the proxy ownKeys call count.

  The first four fail on master.

• 12.6.4-2.js and order-enumerable-shadowed.js now pass, so I removed them from test262.properties.

./gradlew check passes. The one exception is JLineConsoleTest.barePrintlnWritesOnlyLineSeparator, which also fails on master on my Windows machine.