fix(dataflow): Dart arrow-body functions never record an implicit return

[carlos-alm]

Summary

• tree-sitter-dart represents an arrow-bodied function (int f() => x + 1;) as function_body containing the expression DIRECTLY — no return_statement node at all. dataflowDart's returnNode: 'return_statement' never matched, so arrow-bodied functions never recorded a returns entry — a distinct semantic gap from Dart: function_signature/method_signature never contains its own body — breaks per-function complexity/dataflow analysis #2182's sibling-body fix (an implicit return via => isn't modeled as a return at all).
• Adds a new implicitReturnBodyNode/blockBodyNode config pair to DataflowRulesConfig: when a configured node's only named child is not the block type, that child is recorded as an implicit return the same way handleReturn already does for a real return_statement (it only ever reads namedChildren[0], so it works unmodified regardless of the wrapper node's actual type).
• Enabled only for Dart (implicitReturnBodyNode: 'function_body') — no other language sets this field, so this is purely additive with zero behavior change elsewhere.
• Rust's native engine has no Dart dataflow rules at all yet (tracked separately as follow-up: Rust engine needs DartDataflowRules to match TS's now-fixed dataflowDart #2359), so this is legitimately TS/WASM-only — no dual-engine parity gap for this specific fix.
• While verifying, found that calls nested inside a return/other expression are never tracked at all for Dart (argFlows stays empty even for an ordinary block-bodied function) — a separate, pre-existing gap, unrelated to this fix. Filed as follow-up: Dart dataflow — calls nested inside a return/other expression are never tracked #2497.

Test plan

• New tests in tests/parsers/dataflow-dart.test.ts: implicit return from a top-level arrow-bodied function, from an arrow-bodied class method, and a check that a block-bodied function isn't double-counted
• Full test suite: 328 files / 5263 tests passed
• npm run lint clean
• diff-impact — contained (1 function changed, 1 transitive caller)

Closes #2356

[greptile-apps]

Greptile Summary

The PR adds Dart expression-bodied function support to the TypeScript/WASM dataflow analyzer.

• Extends DataflowRulesConfig with configurable implicit-return and block-body node types.
• Configures Dart function_body nodes to produce return entries when their sole named child is an expression rather than a block.
• Adds coverage for top-level functions, class methods, and block-body duplicate prevention.

Confidence Score: 5/5

The PR appears safe to merge with no concrete correctness or security issues identified.

The Dart-only configuration recognizes expression-bodied functions without changing other language rules, while the visitor still traverses expression children and avoids treating block bodies as implicit returns.

Important Files Changed

Filename	Overview
src/ast-analysis/visitors/dataflow-visitor.ts	Adds narrowly configured implicit-return dispatch while preserving traversal of nested expression nodes.
src/ast-analysis/rules/b2.ts	Enables implicit-return body recognition only for Dart and retains block as the statement-body discriminator.
src/ast-analysis/shared.ts	Adds inert defaults for the new dataflow configuration fields, leaving other languages unchanged.
src/types.ts	Documents and types the new implicit-return configuration contract consistently with its defaults and use.
tests/parsers/dataflow-dart.test.ts	Covers top-level and method arrow bodies plus prevention of duplicate returns for block bodies.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Dart function_body] --> B{Exactly one named child?}
  B -->|No| E[Continue normal child traversal]
  B -->|Yes| C{Child type is block?}
  C -->|Yes| E
  C -->|No| D[Record child expression as implicit return]
  D --> E

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[github-actions]

Codegraph Impact Analysis

1 functions changed → 1 callers affected across 1 files

• dispatchDataflowNode in src/ast-analysis/visitors/dataflow-visitor.ts:481 (1 transitive callers)