anchor.diff:  
     /// Maps Kernel constants to their JS aliases.
     final constAliasCache = HashMap<Constant, js_ast.Expression>();
  +
  +  /// Maps uri strings in asserts and elsewhere to hoisted identifiers.
  +  final _uriMap = HashMap<String, js_ast.Identifier>();
   
     final Class _jsArrayClass;
     final Class _privateSymbolClass;
  
positive.code:     throw UnsupportedError('compilation of an assert block');
    }
  
    @override
    js_ast.Statement visitAssertStatement(AssertStatement node) {
      if (!_options.enableAsserts) return js_ast.EmptyStatement();
  
negative.code:         js_ast.LiteralNull()
        else
          _visitExpression(node.message),
        _cacheUri(location.sourceUrl.toString()),
        // Lines and columns are typically printed with 1 based indexing.
        js.number(location.line + 1),
        js.number(location.column + 1),
  