Add a Dart runtime for Protovalidate

[SpencerC]

Adds a first Dart implementation of Protovalidate (see #148).

Approach

Reflective, descriptor-driven runtime that mirrors the Go/Python/ES design: rules and structure are read from a FileDescriptorSet (with the buf.validate extensions registered), and field values are read from generated messages via reflection. The same core powers both the Validator API and the conformance executor. Because Dart has no dynamicpb, the executor unpacks each Any using a generated registry of the conformance case types.

Scope

Implements the standard, CEL-free subset of Protovalidate:

• scalars — const / lt / lte / gt / gte / in / not_in / finite, including combined ranges (gt_lt, gte_lte_exclusive, …)
• string (including email / uri / uuid), bytes, bool, enum
• repeated and map, including items / keys / values
• required / ignore and nested-message recursion

Out of scope for now (full parity would require a Dart CEL evaluator): custom / predefined / message-level CEL rules, well-known-type rules, additional string/bytes formats (hostname, ip*, address, uri_ref, tuuid, ulid, host_and_port, …), and some Protobuf Editions cases (a few of which protoc-gen-dart does not yet generate). These are enumerated in dart/conformance/expected_failures.yaml.

Conformance

Passes every in-subset case with no unexpected failures:

make conformance-dart
PASS (failed: 0, skipped: 778, passed: 2094, total: 2872)

(The 778 "skipped" are the out-of-subset cases listed in expected_failures.yaml.)

What's included

• dart/ — the protovalidate package, conformance executor, unit tests, and README
• Makefile — test-dart, conformance-dart, generate-dart

Notes for maintainers

• Kept in-repo under dart/ for now so it can reuse the conformance harness; happy to extract to a protovalidate-dart repo if that's preferred.
• No CI wiring added yet — a dart job running make test-dart + make conformance-dart can be added on request.
• Opening as a draft: feedback welcome on the architecture and the subset scope before further polish.

🤖 Generated with Claude Code