fix(dart): keep top-level fromJson/toJson names with --from-map

[schani]

Bug

With --from-map (methodNamesWithMap), quicktype's Dart output renamed the
top-level free functions sensordataFromJson/sensordataToJson to
sensordataFromMap/sensordataToMap, even though those top-level functions
take/return a JSON String (via json.decode/json.encode), not a Map.
Only the class-level factory constructor and instance method — which really
do operate on Map<String, dynamic> — should change name when --from-map
is used.

Repro:

node dist/index.js -l dart --from-map --top-level Sensordata sensordata.json

Before the fix:

Sensordata sensordataFromMap(String str) => Sensordata.fromMap(json.decode(str));
String sensordataToMap(Sensordata data) => json.encode(data.toMap());

After the fix:

Sensordata sensordataFromJson(String str) => Sensordata.fromMap(json.decode(str));
String sensordataToJson(Sensordata data) => json.encode(data.toMap());

Root cause

DartRenderer.makeTopLevelDependencyNames built the top-level encoder/decoder
names using the toJson/fromJson getters, which are conditional on
methodNamesWithMap and return toMap/fromMap when the option is set.
Those getters are also (correctly) used for the class-level
fromMap/toMap methods emitted by _emitMapEncoderDecoder, so the
top-level functions inherited the same renaming even though they don't
operate on a Map.

Fix

makeTopLevelDependencyNames now hard-codes the _toJson/_fromJson
suffixes for the top-level function names, independent of
methodNamesWithMap. The class-level Map methods are untouched and keep
using the conditional naming, matching the fix suggested by a maintainer in
the issue thread.

Test coverage

• test/unit/dart-method-names.test.ts (new): renders Dart both with and
  without the from-map renderer option and asserts the top-level functions
  are always named ..FromJson/..ToJson while the class-level
  factory/method are named fromMap/toMap only when the option is set
  (and fromJson/toJson otherwise, i.e. no regression to the default
  case).
• test/languages.ts: added a from-map renderer-option variant to the Dart
  fixture's quickTestRendererOptions (using simple-object.json) so the
  end-to-end fixture also exercises code generation with this option.

Verification

• Confirmed the bug on unfixed master with the CLI repro above.
• Added the failing unit test first, confirmed it failed against the
  unfixed code, then applied the fix and confirmed it passes.
• npm run build passes.
• npm run test:unit — 165/165 tests pass.
• Re-ran the CLI repro manually; output now matches the expected
  fromJson/toJson top-level names while class-level fromMap/toMap
  names are preserved, and confirmed no regression when --from-map is
  omitted.
• The Dart toolchain (dart binary) is not available in this sandboxed
  environment, so the FIXTURE=dart script/test end-to-end run itself
  wasn't executed locally; the new from-map fixture variant will be
  validated by CI.

Fixes #1719

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