anchor.diff:      var program = <JS.Statement>[
         js.statement("dart_library.library(#, #, #, #, #)", [
           js.string(jsPath, "'"),
  -        jsDefaultValue != null ? jsDefaultValue : new JS.LiteralNull(),
  +        jsDefaultValue ?? new JS.LiteralNull(),
           js.commentExpression(
               "Imports", new JS.ArrayInitializer(imports, multiline: true)),
           js.commentExpression("Lazy imports",
  
positive.code:   @override
    visitBreakStatement(BreakStatement node) {
      var label = node.label;
      return new JS.Break(label != null ? label.name : null);
    }
  
    @override
    visitContinueStatement(ContinueStatement node) {
      var label = node.label;
      return new JS.Continue(label != null ? label.name : null);
    }
  
    @override
  
negative.code:     var value = _visit(node.initializer);
      // explicitly initialize to null, to avoid getting `undefined`.
      // TODO(jmesserly): do this only for vars that aren't definitely assigned.
      return value ?? new JS.LiteralNull();
    }
  
    void _flushLazyFields(List<JS.Statement> body) {
  