feat(rules_dart): first-class Bazel ruleset with RBE, IDE aspects, and version conflict detection

[yesudeep]

Summary

This PR transforms rules_dart into a first-class Bazel ruleset with feature parity to rules_go and rules_rust. It is now ready for BCR (Bazel Central Registry) publication.

Feature Parity Matrix

Category	Feature	Status
Core Rules	dart_library, dart_binary, dart_test, dart_native_binary	✅
Compilation	dart_js_binary, dart_wasm_binary, dart_aot_snapshot	✅
Toolchain	dart_toolchain, DartToolchainInfo, multi-SDK support	✅
Dependencies	deps.bzl, dart_package, version conflict detection	✅
Code Generation	dart_proto_library, dart_grpc_library, dart_build_runner	✅
Testing	Coverage (LCOV), test sharding (shard_count)	✅
Remote Execution	RBE configuration (BuildBuddy, Google RBE)	✅
IDE Integration	dart_ide_info aspect, DartIdeInfo provider	✅
Platform Support	Windows (.bat), macOS, Linux - 100% cross-platform	✅
Performance	Persistent workers (JSON protocol)	✅
Gazelle	Go-based BUILD.bazel generator	✅

Key Changes

Phase 1-2: Foundation & Toolchain

• Cross-platform script generation (100% Windows native, no MSYS)
• Hermetic SDK download
• Persistent workers with JSON protocol
• dart_toolchain rule with DartToolchainInfo provider
• toolchain_type registration for multi-SDK support

Phase 3: Dependency Management

• pubspec.lock parsing in Starlark
• dart_package repository rule
• dart_deps module extension
• Version conflict detection across modules with actionable error messages

Phase 4: Testing & Coverage

• Coverage script generation (Unix/Windows)
• LCOV output format for bazel coverage
• Test sharding via shard_count attribute

Phase 5: Code Generation

• dart_proto_library - Proto message generation
• dart_grpc_library - gRPC service stub generation
• dart_build_runner - freezed/json_serializable support
• Working examples: proto_example/, freezed_example/

Phase 6: Advanced Features

• rbe.bazelrc - Complete RBE configuration
• BuildBuddy and Google Cloud Build support
• Remote caching (read-only and read-write modes)

Phase 8: IDE Integration

• dart_ide_info aspect for IntelliJ/VSCode
• DartIdeInfo provider with source/dependency info
• VSCode settings generator
• IntelliJ facet generator

File Structure

rules_dart/
├── defs.bzl           # Core rules
├── deps.bzl           # Dependency management + conflict detection
├── toolchain.bzl      # Toolchain abstraction
├── proto.bzl          # Proto/gRPC code generation
├── build_runner.bzl   # build_runner integration
├── aspects.bzl        # IDE aspects
├── rbe.bazelrc        # Remote execution config
├── ROADMAP.md         # Feature parity tracking
├── private/
│   ├── coverage.bzl   # Coverage support
│   ├── rbe.bzl        # RBE utilities
│   ├── helpers.bzl    # Common utilities
│   ├── windows.bzl    # Windows script generation
│   └── unix.bzl       # Unix script generation
├── worker/            # Persistent worker
├── gazelle/           # Go-based Gazelle extension
├── scripts/           # Cross-platform test scripts
└── examples/
    ├── hello_world/       # Basic rules
    ├── proto_example/     # Proto/gRPC (with test.sh + test.bat)
    └── freezed_example/   # build_runner (with test.sh + test.bat)

Remaining Future Work

• Flutter Support (Phase 7): flutter_library, flutter_test, flutter_application

Testing

• All core rules tested on Windows, macOS, Linux
• Cross-platform test scripts (.sh + .bat) for all examples
• CI workflow validates portability

Breaking Changes

None - all changes are additive.