dart2js kernel: DeferredImportsTask is slow

[rakudrama]

On a large app, the AST pipeline spends ~2s in DeferredImportsTask, the Kernel pipeline ~20s.

Profiling shows ~8% on the line below starting with .any(.

It seems like collecting the additionalExports into a cached Set would fix the immediate performance problem.

  @override
  Iterable<ImportEntity> importsTo(Entity element, LibraryEntity library) {
    if (element is! MemberEntity) return const <ImportEntity>[];
    List<ImportEntity> imports = [];
    ir.Library source = _elementMap.getLibraryNode(library);
    ir.Member member = _elementMap.getMemberDefinition(element).node;
    for (ir.LibraryDependency dependency in source.dependencies) {
      if (dependency.isExport) continue;
      if (!_isVisible(dependency.combinators, member.name.name)) continue;
      if (member.enclosingLibrary == dependency.targetLibrary ||
          dependency.targetLibrary.additionalExports
              .any((ir.Reference ref) => ref.node == member)) {
        imports.add(_elementMap.getImport(dependency));
      }
    }
    return imports;
  }

[sigmundch]

To document our plan here:

• we'll temporarily do something simple in dart2js to improve performance of this method (e.g. caching of additionalExports as a set)
• later we want to improve deferred loading so that it operates at a finer granularity: we have enough information during resolution so we can record how dependencies exist between elements, so we should need to be using the import graph entirely. One idea we like is to attach to each method some metadata that indicates which entities it depends on, and through which deferred import they were used.

[rakudrama]

Temporary speedup via caching additionalExports: from ~20s to ~5-6s: https://dart-review.googlesource.com/c/sdk/+/25740
This is not as fast as the ~2s for the other version.