anchor.diff:  import "dart:math" as $dart$math;
   import "dart:async" as $dart$async;
   import "dart:collection" as $dart$collection;
  -import "dart:async" as $dart$async;
  -import "dart:collection" as $dart$collection;
  +import "package:ceylon/interop/dart/dart.dart" as $ceylon$interop$dart;
   import "source/ceylon/language/module.dart";
  -import "package:ceylon/interop/dart/dart.dart" as $ceylon$interop$dart;
   
   Callable $package$and([Callable p, Callable q]) => new dart$Callable(([$dart$core.Object $0]) => Boolean.instance((([$dart$core.Object val]) => Boolean.nativeValue(p.f(val) as Boolean) && Boolean.nativeValue(q.f(val) as Boolean))($0)));
   
  
positive.code:     }
      $dart$core.String toString() => ((("" + _$first.toString()) + ".chain(") + _$second.toString()) + ").iterator()";
  }
  class Collection$permutations$$anonymous$0_$$anonymous$5_ implements dart$$Basic, Iterator {
      Collection$permutations$$anonymous$0_ $outer$ceylon$language$Collection$permutations$$anonymous$0_;
      Collection$permutations$$anonymous$0_$$anonymous$5_([Collection$permutations$$anonymous$0_ $outer$ceylon$language$Collection$permutations$$anonymous$0_]) {
  
negative.code: 
  class StringBuilder implements dart$$Basic, List {
      StringBuilder() {
          _$delegate = new $dart$core.StringBuffer();
      }
      $dart$core.StringBuffer _$delegate;
      $dart$core.int get size => String.instance(_$delegate.toString()).size;
      $dart$core.int get lastIndex => !empty ? size - 1 : null;
      $dart$core.String toString() => _$delegate.toString();
  