Dart 3.13 shorthand factory constructors are parsed as unnamed ('new'), crashing PackageApiDiffer #266

When analyzing a package that uses Dart 3.13 primary constructors and shorthand named factory constructor syntax (where the enclosing class prefix is omitted, e.g. factory fromSamples(...) instead of factory BenchmarkMetrics.fromSamples(...)), dart_apitool diff crashes with an ArgumentError:

Error while creating the diff: Invalid argument(s): Multiple new items found with the same structure as the old item ExecutableDeclaration(returnTypeName: BenchmarkMetrics, ..., name: new, ...): (ExecutableDeclaration(..., name: new, parameters: [...]), ExecutableDeclaration(..., name: new, parameters: [samplesNs, ...]), ExecutableDeclaration(..., name: new, parameters: [json]))
#0      PackageApiDiffer._diffIterables (package:dart_apitool/src/diff/package_api_differ.dart:1432:11)
#1      PackageApiDiffer._calculateExecutablesDiff (package:dart_apitool/src/diff/package_api_differ.dart:298:32)

Root Cause

1. dart_apitool currently constrains analyzer: ^10.0.2 (Dart 3.9/3.10 era, predating Dart 3.13 primary and shorthand constructor syntax).
2. Given a class with a primary constructor and shorthand factory constructors:

   final class const BenchmarkMetrics({
     required final double meanNs,
   }) {
     factory fromSamples(List<double> samplesNs) => ...;
     factory fromJson(Map<String, Object?> json) => ...;
   }

   analyzer 10.2.0 parses factory fromSamples(...) and factory fromJson(...) using the pre-3.13 grammar factory <typeIdentifier>(...) (an unnamed constructor with period == null and name == null).
3. Because ConstructorElement.name is 'new' when period == null, InternalExecutableDeclaration.fromExecutableElement emits name: 'new' for the primary constructor and every shorthand factory constructor on the class.
4. PackageApiDiffer._calculateExecutablesDiff then matches executables by oldEx.name == newEx.name, finds multiple constructors named 'new', and throws in _diffIterables (sameMatches.singleOrNull == null).

Resolution

Updating analyzer to ^12.0.0 (or migrating the analyzer frontend to package:api_summary, as explored in #258) resolves the constructor names properly (new, fromSamples, fromJson).