[average-web-dev]

This adds the ES2025 Iterator global and the iterator helper methods, plus
Iterator.concat from the iterator sequencing proposal. It is a fresh
implementation rather than a rebase of #2078, which had conflicts and caused
regressions in unrelated areas of test262.properties.

What's included

• Iterator constructor: abstract (throws a TypeError when called or
  constructed directly), with Iterator.prototype non-writable and
  non-configurable.
• Iterator.from, including %WrapForValidIteratorPrototype% for iterators
  that don't inherit from Iterator.prototype.
• Iterator.concat.
• Lazy helpers: map, filter, take, drop, flatMap, implemented as a
  small state machine behind %IteratorHelperPrototype%. They follow the
  generator state semantics: a reentrant next() throws, and return() behaves
  correctly in the suspended-start and suspended-yield states.
• Eager helpers: reduce, toArray, forEach, some, every, find.
• Iterator.prototype.constructor and [Symbol.toStringTag] as accessor
  properties, using SetterThatIgnoresPrototypeProperties.
• Spec-ordered argument validation: the underlying iterator is closed when
  validation fails, next is read only once (GetIteratorDirect), and
  IteratorClose keeps its throw and normal completion semantics.

Shared %Iterator.prototype%

All built-in iterators (Array, String, Map, Set, RegExp String, Java
iterators) and generators now inherit from a common %Iterator.prototype%.
Before this change, each iterator prototype defined its own
[Symbol.iterator] and inherited directly from Object.prototype.

Language version instead of a feature flag

Following the discussion in #2078, the new Iterator is installed only for
Context.VERSION_ECMASCRIPT, with no feature flag. Older language versions
keep the legacy JS 1.7 Iterator and see no behavior change. For those
versions only a hidden %Iterator.prototype% is created, so that built-in
iterators still share a common prototype.

test262

built-ins/Iterator: 465/510 → 243/510 failing. No new failures in any
other directory.

The remaining failures are outside the scope of this PR:

Reason	Tests
Iterator.zip / zipKeyed (not yet in the spec)	73
Test uses class … extends Iterator (class syntax is unsupported)	144
Primitive this gets boxed, since Callable.call takes a Scriptable receiver	15
Symbol.dispose (explicit resource management)	6
Multiple spreads in a single call crash codegen (f(...a, ...b)), an existing bug	1
Cross-realm ($262) / subclassable.js	2

Tests

New JUnit 5 class tests/.../es2025/IteratorHelpersTest.java. It covers the
prototype chain, the abstract constructor, each helper, laziness, closing of
the underlying iterator on early exit and on validation errors, reentrancy,
from, concat, the accessor properties, and unchanged legacy behavior below
VERSION_ECMASCRIPT.

SerializationCoverageTest is updated for the new serializable root
(IteratorRecord).

A note on how this was developed

I developed this PR with the help of an AI coding agent (Claude Code), and I
reviewed, tested and take responsibility for all of the changes. If the project
has concerns about or rules for AI-assisted contributions, please let me know.
I'm happy to rework parts of it, split it up, or change my approach.

I'm dyslexic, so I usually let AI help me write or proofread text like this
description. If anything reads unclear, please ask and I'll clarify.

🤖 Generated with Claude Code