// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

library dart_tree_printer;

import '../constants/values.dart' as values;
import '../dart_types.dart' as types;
import '../diagnostics/invariant.dart' show
    invariant;
import '../elements/elements.dart' as elements;
import '../resolution/tree_elements.dart' show
    TreeElementMapping;
import '../tokens/token.dart';
import '../tokens/token_constants.dart';
import '../tokens/precedence.dart';
import '../tokens/precedence_constants.dart';
import '../tree/tree.dart' as tree;
import '../util/util.dart';
import 'backend_ast_nodes.dart';
import 'backend_ast_emitter.dart' show TypeGenerator;

/// Translates the backend AST to Dart frontend AST.
tree.Node emit(TreeElementMapping treeElements,
               RootNode root) {
  return new TreePrinter(treeElements).makeDefinition(root);
}

/// If true, the unparser will insert a coment in front of every function
/// it emits. This helps indicate which functions were translated by the new
/// backend.
bool INSERT_NEW_BACKEND_COMMENT =
    const bool.fromEnvironment("INSERT_NEW_BACKEND_COMMENT");

/// Converts backend ASTs to frontend ASTs.
class TreePrinter {
  TreeElementMapping treeElements;

  TreePrinter([this.treeElements]);

  tree.Node makeDefinition(RootNode node) {
    if (node is FieldDefinition) {
      tree.Node definition;
      if (node.initializer == null) {
        definition = makeIdentifier(node.element.name);
      } else {
        definition = new tree.SendSet(
            null,
            makeIdentifier(node.element.name),
            new tree.Operator(assignmentToken("=")),
            singleton(makeExpression(node.initializer)));
      }
      setElement(definition, node.element, node);
      return new tree.VariableDefinitions(
          null, // TODO(sigurdm): Type
          makeVarModifiers(useVar: true,
                           isFinal: node.element.isFinal,
                           isStatic: node.element.isStatic,
                           isConst: node.element.isConst),
          makeList(null, [definition], close: semicolon));
    } else if (node is FunctionExpression) {
      return makeExpression(node);
    } else {
      assert(false);
      return null;
    }
  }

  void setElement(tree.Node node, elements.Element element, source) {
    if (treeElements != null) {
      if (element == null) {
        throw "Missing element from ${source}";
      }
      treeElements[node] = element;
    }
  }

  void setType(tree.Node node, types.DartType type, source) {
    if (treeElements != null) {
      if (type == null) {
        throw "Missing type from ${source}";
      }
      treeElements.setType(node, type);
    }
  }

  // Group tokens: () [] {} <>
  static BeginGroupToken makeGroup(PrecedenceInfo open, PrecedenceInfo close) {
    BeginGroupToken openTok = new BeginGroupToken(open, -1);
    openTok.endGroup = new SymbolToken(close, -1);
    return openTok;
  }

  final BeginGroupToken openParen = makeGroup(OPEN_PAREN_INFO,
                                              CLOSE_PAREN_INFO);
  final BeginGroupToken openBrace = makeGroup(OPEN_CURLY_BRACKET_INFO,
                                              CLOSE_CURLY_BRACKET_INFO);
  final BeginGroupToken openBracket = makeGroup(OPEN_SQUARE_BRACKET_INFO,
                                                CLOSE_SQUARE_BRACKET_INFO);
  final BeginGroupToken lt = makeGroup(LT_INFO, GT_INFO);

  Token get closeParen => openParen.endGroup;
  Token get closeBrace => openBrace.endGroup;
  Token get closeBracket => openBracket.endGroup;
  Token get gt => lt.endGroup;

  // Symbol tokens
  final Token semicolon = new SymbolToken(SEMICOLON_INFO, -1);
  final Token indexToken = new SymbolToken(INDEX_INFO, -1); // "[]"
  final Token question = new SymbolToken(QUESTION_INFO, -1);
  final Token colon = new SymbolToken(COLON_INFO, -1);
  final Token hash = new SymbolToken(HASH_INFO, -1);
  final Token bang = new SymbolToken(BANG_INFO, -1);
  final Token eq = new SymbolToken(EQ_INFO, -1);

  // Keyword tokens
  static Token makeIdToken(String text) {
    return new StringToken.fromString(IDENTIFIER_INFO, text, -1);
  }
  final Token newToken = makeIdToken('new');
  final Token constToken = makeIdToken('const');
  final Token throwToken = makeIdToken('throw');
  final Token rethrowToken = makeIdToken('rethrow');
  final Token breakToken = makeIdToken('break');
  final Token continueToken = makeIdToken('continue');
  final Token doToken = makeIdToken('do');
  final Token whileToken = makeIdToken('while');
  final Token ifToken = makeIdToken('if');
  final Token elseToken = makeIdToken('else');
  final Token awaitToken = makeIdToken('await');
  final Token forToken = makeIdToken('for');
  final Token inToken = makeIdToken('in');
  final Token returnToken = makeIdToken('return');
  final Token switchToken = makeIdToken('switch');
  final Token caseToken = makeIdToken('case');
  final Token defaultToken = makeIdToken('default');
  final Token tryToken = makeIdToken('try');
  final Token catchToken = makeIdToken('catch');
  final Token onToken = makeIdToken('on');
  final Token finallyToken = makeIdToken('finally');
  final Token getToken = makeIdToken('get');
  final Token setToken = makeIdToken('set');
  final Token classToken = makeIdToken('class');
  final Token extendsToken = makeIdToken('extends');
  final Token withToken = makeIdToken('with');
  final Token implementsToken = makeIdToken('implements');
  final Token typedefToken = makeIdToken('typedef');
  final Token enumToken = makeIdToken('enum');

  static tree.Identifier makeIdentifier(String name) {
    return new tree.Identifier(
        new StringToken.fromString(IDENTIFIER_INFO, name, -1));
  }

  static tree.Operator makeOperator(String name) {
    return new tree.Operator(
        new StringToken.fromString(IDENTIFIER_INFO, name, -1));
  }

  // Utilities for creating NodeLists
  Link<tree.Node> makeLink(Iterable<tree.Node> nodes) {
    LinkBuilder builder = new LinkBuilder();
    for (tree.Node node in nodes) {
      builder.addLast(node);
    }
    return builder.toLink();
  }

  tree.NodeList blankList() {
    return new tree.NodeList(null, makeLink([]), null, '');
  }
  tree.NodeList singleton(tree.Node node) {
    return new tree.NodeList(null, makeLink([node]), null, '');
  }
  tree.NodeList makeList(String delimiter,
                         Iterable<tree.Node> nodes,
                         { Token open,
                           Token close }) {
    return new tree.NodeList(open, makeLink(nodes), close, delimiter);
  }
  tree.NodeList parenList(String delimiter, Iterable<tree.Node> nodes) {
    return makeList(delimiter, nodes, open: openParen, close: closeParen);
  }
  tree.NodeList bracketList(String delimiter, Iterable<tree.Node> nodes) {
    return makeList(delimiter, nodes, open: openBracket, close: closeBracket);
  }
  tree.NodeList braceList(String delimiter, Iterable<tree.Node> nodes) {
    return makeList(delimiter, nodes, open: openBrace, close: closeBrace);
  }
  tree.NodeList argList(Iterable<tree.Node> nodes) {
    return parenList(',', nodes);
  }
  tree.NodeList typeArgList(Iterable<tree.Node> nodes) {
    return makeList(',', nodes, open: lt, close: gt);
  }

  /// Converts a qualified name into nested Sends.
  tree.Node makeName(String name) {
    if (name == null) {
      return null;
    }
    List<String> names = name.split('.').toList(growable:false);
    tree.Node node = makeIdentifier(names[0]);
    for (int i = 1; i < names.length; i++) {
      node = new tree.Send(node, makeIdentifier(names[i]));
    }
    return node;
  }

  static Token assignmentToken(String operatorName) {
    switch (operatorName) {
      case '=': return new SymbolToken(EQ_INFO, -1);
      case '+=': return new SymbolToken(PLUS_EQ_INFO, -1);
      case '-=': return new SymbolToken(MINUS_EQ_INFO, -1);
      case '*=': return new SymbolToken(STAR_EQ_INFO, -1);
      case '/=': return new SymbolToken(SLASH_EQ_INFO, -1);
      case '~/=': return new SymbolToken(TILDE_SLASH_EQ_INFO, -1);
      case '%=': return new SymbolToken(PERCENT_EQ_INFO, -1);
      case '&=': return new SymbolToken(AMPERSAND_EQ_INFO, -1);
      case '^=': return new SymbolToken(CARET_EQ_INFO, -1);
      case '|=': return new SymbolToken(BAR_EQ_INFO, -1);
      case '>>=': return new SymbolToken(GT_GT_EQ_INFO, -1);
      case '<<=': return new SymbolToken(LT_LT_EQ_INFO, -1);
      default:
        throw "Unrecognized assignment operator: $operatorName";
    }
  }

  static Token binopToken(String operatorName) {
    switch (operatorName) {
      case '+': return new SymbolToken(PLUS_INFO, -1);
      case '-': return new SymbolToken(MINUS_INFO, -1);
      case '*': return new SymbolToken(STAR_INFO, -1);
      case '/': return new SymbolToken(SLASH_INFO, -1);
      case '~/': return new SymbolToken(TILDE_SLASH_INFO, -1);
      case '%': return new SymbolToken(PERCENT_INFO, -1);
      case '&': return new SymbolToken(AMPERSAND_INFO, -1);
      case '^': return new SymbolToken(CARET_INFO, -1);
      case '|': return new SymbolToken(BAR_INFO, -1);
      case '>>': return new SymbolToken(GT_GT_INFO, -1);
      case '<<': return new SymbolToken(LT_LT_INFO, -1);
      case '==': return new SymbolToken(EQ_EQ_INFO, -1);
      case '!=': return new SymbolToken(BANG_EQ_INFO, -1);
      case '>': return new SymbolToken(GT_INFO, -1);
      case '>=': return new SymbolToken(GT_EQ_INFO, -1);
      case '<': return new SymbolToken(LT_INFO, -1);
      case '<=': return new SymbolToken(LT_EQ_INFO, -1);
      case '&&': return new SymbolToken(AMPERSAND_AMPERSAND_INFO, -1);
      case '||': return new SymbolToken(BAR_BAR_INFO, -1);
      default:
        throw "Unrecognized binary operator: $operatorName";
    }
  }

  static Token incrementToken(String operatorName) {
    switch (operatorName) {
      case '++': return new SymbolToken(PLUS_PLUS_INFO, -1);
      case '--': return new SymbolToken(MINUS_MINUS_INFO, -1);
      default:
        throw "Unrecognized increment operator: $operatorName";
    }
  }

  static Token typeOpToken(String operatorName) {
    switch (operatorName) { // "is!" is not an operator in the frontend AST.
      case 'is': return new SymbolToken(IS_INFO, -1);
      case 'as': return new SymbolToken(AS_INFO, -1);
      default:
        throw 'Unrecognized type operator: $operatorName';
    }
  }

  Token unopToken(String operatorName) {
    switch (operatorName) {
      case '-': return new SymbolToken(MINUS_INFO, -1);
      case '~': return new SymbolToken(TILDE_INFO, -1);
      case '!': return bang;
      default:
        throw "Unrecognized unary operator: $operatorName";
    }
  }

  tree.Node makeStaticReceiver(elements.Element element) {
    if (treeElements == null) return null;
    if (element.isStatic) {
      elements.ClassElement enclosingClass = element.enclosingClass;
      tree.Send send = new tree.Send(
          null,
          makeIdentifier(enclosingClass.name));
      treeElements[send] = enclosingClass;
      return send;
    } else {
      return null;
    }
  }

  tree.Node makeArgument(Argument arg) {
    if (arg is Expression) {
      return makeExpression(arg);
    } else if (arg is NamedArgument) {
      return new tree.NamedArgument(
          makeIdentifier(arg.name),
          colon,
          makeExpression(arg.expression));
    } else {
      throw "Unrecognized argument type: ${arg}";
    }
  }

  tree.Node makeExpression(Expression exp) {
    return makeExp(exp, EXPRESSION);
  }

  /// Converts [exp] to a [tree.Node] that unparses to an expression with
  /// a precedence level of at least [minPrecedence]. The expression will be
  /// wrapped in a parenthesis if necessary.
  tree.Node makeExp(Receiver exp, int minPrecedence, {bool beginStmt: false}) {
    tree.Node result;
    int precedence;
    bool needParen = false;
    if (exp is SuperReceiver) {
      precedence = CALLEE;
      result = makeIdentifier('super');
    } else if (exp is Assignment) {
      Expression left = exp.left;
      tree.Node receiver;
      tree.Node selector;
      tree.NodeList arguments;
      elements.Element element;
      if (left is Identifier) {
        receiver = makeStaticReceiver(left.element);
        selector = makeIdentifier(left.name);
        arguments = singleton(makeExpression(exp.right));
        element = left.element;
      } else if (left is FieldExpression) {
        receiver = makeExp(left.object, PRIMARY, beginStmt: beginStmt);
        selector = makeIdentifier(left.fieldName);
        arguments = singleton(makeExpression(exp.right));
      } else if (left is IndexExpression) {
        receiver = makeExp(left.object, PRIMARY, beginStmt: beginStmt);
        selector = new tree.Operator(indexToken);
        arguments = bracketList(',',
            [makeExpression(left.index), makeExpression(exp.right)]);
      } else {
        throw "Unexpected left-hand side of assignment: ${left}";
      }
      tree.Operator op = new tree.Operator(assignmentToken(exp.operator));
      result = new tree.SendSet(receiver, selector, op, arguments);
      if (left is Identifier) {
        setElement(result, element, exp);
      }
      precedence = EXPRESSION;
    } else if (exp is FieldInitializer) {
      precedence = EXPRESSION;
      tree.Node receiver = makeIdentifier('this');
      tree.Node selector = makeIdentifier(exp.element.name);
      tree.Operator op = new tree.Operator(assignmentToken("="));
      // We pass CALLEE to ensure we write eg.:
      // class B { var x; B() : x = (() {return a;}) {}}
      // Not the invalid:
      // class B { var x; B() : x = () {return a;} {}}
      result = new tree.SendSet(receiver, selector, op,
                                singleton(makeExp(exp.body, CALLEE)));
      setElement(result, exp.element, exp);
    } else if (exp is SuperInitializer) {
      precedence = EXPRESSION;
      tree.Node receiver = makeIdentifier('super');
      tree.NodeList arguments =
          argList(exp.arguments.map(makeArgument).toList());
      if (exp.target.name == "") {
        result = new tree.Send(null, receiver, arguments);
      } else {
        result = new tree.Send(receiver,
                               makeIdentifier(exp.target.name),
                               arguments);
      }
      setElement(result, exp.target, exp);
    } else if (exp is BinaryOperator) {
      precedence = BINARY_PRECEDENCE[exp.operator];
      int deltaLeft = isAssociativeBinaryOperator(precedence) ? 0 : 1;
      result = new tree.Send(
          makeExp(exp.left, precedence + deltaLeft, beginStmt: beginStmt),
          new tree.Operator(binopToken(exp.operator)),
          singleton(makeExp(exp.right, precedence + 1)));
    } else if (exp is CallFunction) {
      precedence = CALLEE;
      tree.Node selector;
      Expression callee = exp.callee;
      elements.Element element;
      tree.Node receiver;
      if (callee is Identifier) {
        receiver = makeStaticReceiver(callee.element);
        selector = makeIdentifier(callee.name);
        element = callee.element;
      } else {
        selector = makeExp(callee, CALLEE, beginStmt: beginStmt);
      }
      result = new tree.Send(
          receiver,
          selector,
          argList(exp.arguments.map(makeArgument)));
      if (callee is Identifier) {
        setElement(result, element, exp);
      }
    } else if (exp is CallMethod) {
      precedence = CALLEE;
      // TODO(sra): Elide receiver when This, but only if not in a scope that
      // shadows the method (e.g. constructor body).
      tree.Node receiver = makeExp(exp.object, PRIMARY, beginStmt: beginStmt);
      result = new tree.Send(
          receiver,
          makeIdentifier(exp.methodName),
          argList(exp.arguments.map(makeArgument)));
    } else if (exp is CallNew) {
      precedence = CALLEE;
      tree.Node selector = makeName(exp.type.name);
      if (exp.type.typeArguments.length > 0) {
        selector = new tree.TypeAnnotation(
            selector,
            typeArgList(exp.type.typeArguments.map(makeType)));
        setType(selector, exp.dartType, exp);
      }
      if (exp.constructorName != null) {
        selector = new tree.Send(
            selector,
            makeIdentifier(exp.constructorName));
      }
      tree.Send send = new tree.Send(
          null,
          selector,
          argList(exp.arguments.map(makeArgument)));
      result = new tree.NewExpression(
          exp.isConst ? constToken : newToken,
          send);
      setType(result, exp.dartType, exp);
      setElement(send, exp.constructor, exp);
    } else if (exp is CallStatic) {
      precedence = CALLEE;
      result = new tree.Send(
          makeStaticReceiver(exp.element),
          makeIdentifier(exp.methodName),
          argList(exp.arguments.map(makeArgument)));
      setElement(result, exp.element, exp);
    } else if (exp is Conditional) {
      precedence = CONDITIONAL;
      result = new tree.Conditional(
          makeExp(exp.condition, LOGICAL_OR, beginStmt: beginStmt),
          makeExp(exp.thenExpression, EXPRESSION),
          makeExp(exp.elseExpression, EXPRESSION),
          question,
          colon);
    } else if (exp is FieldExpression) {
      precedence = PRIMARY;
      // TODO(sra): Elide receiver when This, but only if not in a scope that
      // shadows the method (e.g. constructor body).
      tree.Node receiver = makeExp(exp.object, PRIMARY, beginStmt: beginStmt);
      result = new tree.Send(receiver, makeIdentifier(exp.fieldName));
    } else if (exp is ConstructorDefinition) {
      precedence = EXPRESSION;
      tree.NodeList parameters = makeParameters(exp.parameters);
      tree.NodeList initializers =
          exp.initializers == null || exp.initializers.isEmpty
          ? null
          : makeList(",", exp.initializers.map(makeExpression).toList());
      tree.Node body = exp.isConst || exp.body == null
          ? new tree.EmptyStatement(semicolon)
          : makeFunctionBody(exp.body);
      result = new tree.FunctionExpression(constructorName(exp),
          parameters,
          body,
          null,  // return type
          makeFunctionModifiers(exp),
          initializers,
          null,  // get/set
          null); // async modifier
      setElement(result, exp.element, exp);
    } else if (exp is FunctionExpression) {
      precedence = PRIMARY;
      if (beginStmt && exp.name != null) {
        needParen = true; // Do not mistake for function declaration.
      }
      Token getOrSet = exp.isGetter
          ? getToken
          : exp.isSetter
              ? setToken
              : null;
      tree.NodeList parameters = exp.isGetter
          ? makeList("", [])
          : makeParameters(exp.parameters);
      tree.Node body = makeFunctionBody(exp.body);
      result = new tree.FunctionExpression(
          functionName(exp),
          parameters,
          body,
          exp.returnType == null || exp.element.isConstructor
            ? null
            : makeType(exp.returnType),
          makeFunctionModifiers(exp),
          null,  // initializers
          getOrSet,  // get/set
          null);  // async modifier
      elements.Element element = exp.element;
      if (element != null) setElement(result, element, exp);
    } else if (exp is Identifier) {
      precedence = CALLEE;
      result = new tree.Send(
          makeStaticReceiver(exp.element),
          makeIdentifier(exp.name));
      setElement(result, exp.element, exp);
    } else if (exp is Increment) {
      Expression lvalue = exp.expression;
      tree.Node receiver;
      tree.Node selector;
      tree.Node argument;
      bool innerBeginStmt = beginStmt && !exp.isPrefix;
      if (lvalue is Identifier) {
        receiver = makeStaticReceiver(lvalue.element);
        selector = makeIdentifier(lvalue.name);
      } else if (lvalue is FieldExpression) {
        receiver = makeExp(lvalue.object, PRIMARY, beginStmt: innerBeginStmt);
        selector = makeIdentifier(lvalue.fieldName);
      } else if (lvalue is IndexExpression) {
        receiver = makeExp(lvalue.object, PRIMARY, beginStmt: innerBeginStmt);
        selector = new tree.Operator(indexToken);
        argument = makeExpression(lvalue.index);
      } else {
        throw "Unrecognized left-hand side: ${lvalue}";
      }
      tree.Operator op = new tree.Operator(incrementToken(exp.operator));
      if (exp.isPrefix) {
        precedence = UNARY;
        result = new tree.SendSet.prefix(receiver, selector, op, argument);
      } else {
        precedence = POSTFIX_INCREMENT;
        result = new tree.SendSet.postfix(receiver, selector, op, argument);
      }
      if (lvalue is Identifier) {
        setElement(result, lvalue.element, exp);
      }
    } else if (exp is IndexExpression) {
      precedence = CALLEE;
      result = new tree.Send(
          makeExp(exp.object, PRIMARY, beginStmt: beginStmt),
          new tree.Operator(indexToken),
          bracketList(',', [makeExpression(exp.index)]));
    } else if (exp is Literal) {
      precedence = CALLEE;
      values.PrimitiveConstantValue value = exp.value;
      Token tok = new StringToken.fromString(
          STRING_INFO, '${value.primitiveValue}', -1);
      if (value.isString) {
        result = unparseStringLiteral(exp);
      } else if (value.isInt) {
        result = new tree.LiteralInt(tok, null);
      } else if (value.isDouble) {
        if (value.primitiveValue == double.INFINITY) {
          precedence = MULTIPLICATIVE;
          tok = new StringToken.fromString(STRING_INFO, '1/0.0', -1);
        } else if (value.primitiveValue == double.NEGATIVE_INFINITY) {
          precedence = MULTIPLICATIVE;
          tok = new StringToken.fromString(STRING_INFO, '-1/0.0', -1);
        } else if (value.primitiveValue.isNaN) {
          precedence = MULTIPLICATIVE;
          tok = new StringToken.fromString(STRING_INFO, '0/0.0', -1);
        }
        result = new tree.LiteralDouble(tok, null);
      } else if (value.isBool) {
        result = new tree.LiteralBool(tok, null);
      } else if (value.isNull) {
        result = new tree.LiteralNull(tok);
      } else {
        throw "Unrecognized constant: ${value}";
      }
    } else if (exp is LiteralList) {
      precedence = PRIMARY;
      tree.NodeList typeArgs = null;
      if (exp.typeArgument != null) {
        typeArgs = typeArgList([makeType(exp.typeArgument)]);
      }
      result = new tree.LiteralList(
          typeArgs,
          bracketList(',', exp.values.map(makeExpression)),
          exp.isConst ? constToken : null);
    } else if (exp is LiteralMap) {
      precedence = PRIMARY;
      if (beginStmt) {
        // The opening brace can be confused with a block statement.
        needParen = true;
      }
      tree.NodeList typeArgs = null;
      if (exp.typeArguments != null && exp.typeArguments.length > 0) {
        typeArgs = typeArgList(exp.typeArguments.map(makeType));
      }
      result = new tree.LiteralMap(
          typeArgs,
          braceList(',', exp.entries.map(makeLiteralMapEntry)),
          exp.isConst ? constToken : null);
    } else if (exp is LiteralSymbol) {
      precedence = PRIMARY;
      result = new tree.LiteralSymbol(
          hash,
          makeList('.', exp.id.split('.').map(makeIdentifier)));
    } else if (exp is LiteralType) {
      precedence = TYPE_LITERAL;
      elements.Element optionalElement = exp.type.element;
      result = new tree.Send(
          optionalElement == null ? null : makeStaticReceiver(optionalElement),
          makeIdentifier(exp.name));
      treeElements.setType(result, exp.type);
      if (optionalElement != null) { // dynamic does not have an element
        setElement(result, optionalElement, exp);
      }
    } else if (exp is ReifyTypeVar) {
      precedence = PRIMARY;
      result = new tree.Send(
          null,
          makeIdentifier(exp.name));
      setElement(result, exp.element, exp);
      setType(result, exp.element.type, exp);
    } else if (exp is StringConcat) {
      precedence = PRIMARY;
      result = unparseStringLiteral(exp);
    } else if (exp is This) {
      precedence = CALLEE;
      result = makeIdentifier('this');
    } else if (exp is Throw) {
      precedence = EXPRESSION; // ???
      result = new tree.Throw(
          makeExpression(exp.expression),
          throwToken,
          throwToken); // endToken not used by unparser
    } else if (exp is TypeOperator) {
      precedence = RELATIONAL;
      tree.Operator operator;
      tree.Node rightOperand = makeType(exp.type);
      if (exp.operator == 'is!') {
        operator = new tree.Operator(typeOpToken('is'));
        rightOperand = new tree.Send(
            rightOperand,
            new tree.Operator(bang),
            blankList());
      } else {
        operator = new tree.Operator(typeOpToken(exp.operator));
      }
      result = new tree.Send(
          makeExp(exp.expression, BITWISE_OR, beginStmt: beginStmt),
          operator,
          singleton(rightOperand));
    } else if (exp is UnaryOperator) {
      precedence = UNARY;
      result = new tree.Send.prefix(
          makeExp(exp.operand, UNARY),
          new tree.Operator(unopToken(exp.operatorName)));
    } else {
      throw "Unknown expression type: ${exp}";
    }

    needParen = needParen || precedence < minPrecedence;
    if (needParen) {
      result = parenthesize(result);
    }
    return result;
  }

  /// Creates a LiteralString with [verbatim] as the value.
  /// No (un)quoting or (un)escaping will be performed by this method.
  /// The [DartString] inside the literal will be set to null because the
  /// code emitter does not use it.
  tree.LiteralString makeVerbatimStringLiteral(String verbatim) {
    Token tok = new StringToken.fromString(STRING_INFO, verbatim, -1);
    return new tree.LiteralString(tok, null);
  }

  tree.LiteralMapEntry makeLiteralMapEntry(LiteralMapEntry en) {
    return new tree.LiteralMapEntry(
        makeExpression(en.key),
        colon,
        makeExpression(en.value));
  }

  /// A comment token to be inserted when [INSERT_NEW_BACKEND_COMMENT] is true.
  final SymbolToken newBackendComment = new SymbolToken(
      const PrecedenceInfo('/* new backend */ ', 0, OPEN_CURLY_BRACKET_TOKEN),
      -1);

  tree.Node makeFunctionBody(Statement stmt) {
    if (INSERT_NEW_BACKEND_COMMENT) {
      return new tree.Block(makeList('', [makeBlock(stmt)],
                                     open: newBackendComment));
    } else {
      return makeBlock(stmt);
    }
  }

  /// Produces a statement in a context where only blocks are allowed.
  tree.Node makeBlock(Statement stmt) {
    if (stmt is Block || stmt is EmptyStatement) {
      return makeStatement(stmt);
    } else {
      return new tree.Block(braceList('', [makeStatement(stmt)]));
    }
  }

  /// Adds the given statement to a block. If the statement itself is a block
  /// it will be flattened (if this does not change lexical scoping).
  void addBlockMember(Statement stmt, List<tree.Node> accumulator) {
    if (stmt is Block && !stmt.statements.any(Unparser.definesVariable)) {
      for (Statement innerStmt in stmt.statements) {
        addBlockMember(innerStmt, accumulator);
      }
    } else if (stmt is EmptyStatement) {
      // No need to include empty statements inside blocks
    } else {
      accumulator.add(makeStatement(stmt));
    }
  }

  /// True if [stmt] is equivalent to an empty statement.
  bool isEmptyStatement(Statement stmt) {
    return stmt is EmptyStatement ||
          (stmt is Block && stmt.statements.every(isEmptyStatement));
  }

  tree.Node makeStatement(Statement stmt, {bool shortIf: true}) {
    if (stmt is Block) {
      List<tree.Node> body = <tree.Node>[];
      for (Statement innerStmt in stmt.statements) {
        addBlockMember(innerStmt, body);
      }
      return new tree.Block(braceList('', body));
    } else if (stmt is Break) {
      return new tree.BreakStatement(
          stmt.label == null ? null : makeIdentifier(stmt.label),
          breakToken,
          semicolon);
    } else if (stmt is Continue) {
      return new tree.ContinueStatement(
          stmt.label == null ? null : makeIdentifier(stmt.label),
          continueToken,
          semicolon);
    } else if (stmt is DoWhile) {
      return new tree.DoWhile(
          makeStatement(stmt.body, shortIf: shortIf),
          parenthesize(makeExpression(stmt.condition)),
          doToken,
          whileToken,
          semicolon);
    } else if (stmt is EmptyStatement) {
      return new tree.EmptyStatement(semicolon);
    } else if (stmt is ExpressionStatement) {
      return new tree.ExpressionStatement(
          makeExp(stmt.expression, EXPRESSION, beginStmt: true),
          semicolon);
    } else if (stmt is For) {
      tree.Node initializer;
      if (stmt.initializer is VariableDeclarations) {
        initializer = makeVariableDeclarations(stmt.initializer);
      } else if (stmt.initializer is Expression) {
        initializer = makeExpression(stmt.initializer);
      } else {
        initializer = null;
      }
      tree.Node condition;
      if (stmt.condition != null) {
        condition = new tree.ExpressionStatement(
            makeExpression(stmt.condition),
            semicolon);
      } else {
        condition = new tree.EmptyStatement(semicolon);
      }
      return new tree.For(
          initializer,
          condition,
          makeList(',', stmt.updates.map(makeExpression)),
          makeStatement(stmt.body, shortIf: shortIf),
          forToken);
    } else if (stmt is ForIn) {
      tree.Node left;
      if (stmt.leftHandValue is Identifier) {
        left = makeExpression(stmt.leftHandValue);
      } else {
        left = makeVariableDeclarations(stmt.leftHandValue);
      }
      return new tree.SyncForIn(
          left,
          makeExpression(stmt.expression),
          makeStatement(stmt.body, shortIf: shortIf),
          forToken,
          inToken);
    } else if (stmt is FunctionDeclaration) {
      tree.FunctionExpression function = new tree.FunctionExpression(
          stmt.name != null ? makeIdentifier(stmt.name) : null,
          makeParameters(stmt.parameters),
          makeFunctionBody(stmt.body),
          stmt.returnType != null ? makeType(stmt.returnType) : null,
          makeEmptyModifiers(),
          null,  // initializers
          null,  // get/set
          null); // async modifier
      setElement(function, stmt.function.element, stmt);
      return new tree.FunctionDeclaration(function);
    } else if (stmt is If) {
      if (stmt.elseStatement == null || isEmptyStatement(stmt.elseStatement)) {
        tree.Node node = new tree.If(
            parenthesize(makeExpression(stmt.condition)),
            makeStatement(stmt.thenStatement),
            null, // else statement
            ifToken,
            null); // else token
        if (shortIf)
          return node;
        else
          return new tree.Block(braceList('', [node]));
      } else {
        return new tree.If(
            parenthesize(makeExpression(stmt.condition)),
            makeStatement(stmt.thenStatement, shortIf: false),
            makeStatement(stmt.elseStatement, shortIf: shortIf),
            ifToken,
            elseToken); // else token
      }
    } else if (stmt is LabeledStatement) {
      List<tree.Label> labels = [];
      Statement inner = stmt;
      while (inner is LabeledStatement) {
        LabeledStatement lbl = inner as LabeledStatement;
        labels.add(new tree.Label(makeIdentifier(lbl.label), colon));
        inner = lbl.statement;
      }
      return new tree.LabeledStatement(
          makeList('', labels),
          makeStatement(inner, shortIf: shortIf));
    } else if (stmt is Rethrow) {
      return new tree.Rethrow(rethrowToken, semicolon);
    } else if (stmt is Return) {
      return new tree.Return(
          returnToken,
          semicolon,
          stmt.expression == null ? null : makeExpression(stmt.expression));
    } else if (stmt is Switch) {
      return new tree.SwitchStatement(
          parenthesize(makeExpression(stmt.expression)),
          braceList('', stmt.cases.map(makeSwitchCase)),
          switchToken);
    } else if (stmt is Try) {
      return new tree.TryStatement(
          makeBlock(stmt.tryBlock),
          makeList(null, stmt.catchBlocks.map(makeCatchBlock)),
          stmt.finallyBlock == null ? null : makeBlock(stmt.finallyBlock),
          tryToken,
          stmt.finallyBlock == null ? null : finallyToken);
    } else if (stmt is VariableDeclarations) {
      return makeVariableDeclarations(stmt, useVar: true, endToken: semicolon);
    } else if (stmt is While) {
      return new tree.While(
          parenthesize(makeExpression(stmt.condition)),
          makeStatement(stmt.body, shortIf: shortIf),
          whileToken);
    } else {
      throw "Unrecognized statement: ${stmt}";
    }
  }

  tree.Node makeVariableDeclaration(VariableDeclaration vd) {
    tree.Node id = makeIdentifier(vd.name);
    setElement(id, vd.element, vd);
    if (vd.initializer == null) {
      return id;
    }
    tree.Node send = new tree.SendSet(
          null,
          id,
          new tree.Operator(eq),
          singleton(makeExpression(vd.initializer)));
    setElement(send, vd.element, vd);
    return send;
  }

  /// If [useVar] is true, the variable definition will use `var` as modifier
  /// if no other modifiers are present.
  /// [endToken] will be used to terminate the declaration list.
  tree.Node makeVariableDeclarations(VariableDeclarations decl,
                                      { bool useVar: false,
                                        Token endToken: null }) {
    return new tree.VariableDefinitions(
        decl.type == null ? null : makeType(decl.type),
        makeVarModifiers(isConst: decl.isConst,
                          isFinal: decl.isFinal,
                          useVar: useVar && decl.type == null),
        makeList(',',
            decl.declarations.map(makeVariableDeclaration),
            close: endToken));
  }

  tree.CatchBlock makeCatchBlock(CatchBlock block) {
    List<tree.VariableDefinitions> formals = [];
    if (block.exceptionVar != null) {
      tree.Node exceptionName = makeIdentifier(block.exceptionVar.name);
      setElement(exceptionName, block.exceptionVar.element, block.exceptionVar);
      formals.add(new tree.VariableDefinitions(
          null,
          makeEmptyModifiers(),
          singleton(exceptionName)));
    }
    if (block.stackVar != null) {
      tree.Node stackTraceName = makeIdentifier(block.stackVar.name);
      setElement(stackTraceName, block.stackVar.element, block.stackVar);
      formals.add(new tree.VariableDefinitions(
          null,
          makeEmptyModifiers(),
          singleton(stackTraceName)));
    }
    return new tree.CatchBlock(
        block.onType == null ? null : makeType(block.onType),
        block.exceptionVar == null ? null : argList(formals),
        makeBlock(block.body),
        block.onType == null ? null : onToken,
        block.exceptionVar == null ? null : catchToken);
  }

  tree.SwitchCase makeSwitchCase(SwitchCase caze) {
    if (caze.isDefaultCase) {
      return new tree.SwitchCase(
          blankList(),
          defaultToken,
          makeList('', caze.statements.map(makeStatement)),
          null); // startToken unused by unparser
    } else {
      return new tree.SwitchCase(
          makeList('', caze.expressions.map(makeCaseMatch)),
          null, // defaultKeyword,
          makeList('', caze.statements.map(makeStatement)),
          null); // startToken unused by unparser
    }
  }

  tree.CaseMatch makeCaseMatch(Expression exp) {
    return new tree.CaseMatch(caseToken, makeExpression(exp), colon);
  }

  tree.TypeAnnotation makeType(TypeAnnotation type) {
    tree.NodeList typeArgs;
    if (type.typeArguments.length > 0) {
      typeArgs = typeArgList(type.typeArguments.map(makeType));
    } else {
      typeArgs = null;
    }
    tree.TypeAnnotation result =
        new tree.TypeAnnotation(makeIdentifier(type.name), typeArgs);
    setType(result, type.dartType, type);
    return result;
  }

  tree.NodeList makeParameters(Parameters params) {
    List<tree.Node> nodes =
        params.requiredParameters.map(makeParameter).toList();
    if (params.hasOptionalParameters) {
      Token assign = params.hasNamedParameters ? colon : eq;
      Token open = params.hasNamedParameters ? openBrace : openBracket;
      Token close = params.hasNamedParameters ? closeBrace : closeBracket;
      Iterable<tree.Node> opt =
          params.optionalParameters.map((p) => makeParameter(p,assign));
      nodes.add(new tree.NodeList(open, makeLink(opt), close, ','));
    }
    return argList(nodes);
  }

  /// [assignOperator] is used for writing the default value.
  tree.Node makeParameter(Parameter param, [Token assignOperator]) {
    if (param.isFunction) {
      tree.Node definition = new tree.FunctionExpression(
          makeIdentifier(param.name),
          makeParameters(param.parameters),
          null, // body
          param.type == null ? null : makeType(param.type),
          makeEmptyModifiers(), // TODO: Function parameter modifiers?
          null,  // initializers
          null,  // get/set
          null); // async modifier
      if (param.element != null) {
        setElement(definition, param.element, param);
      }
      if (param.defaultValue != null) {
        definition = new tree.SendSet(
            null,
            definition,
            new tree.Operator(assignOperator),
            singleton(makeExpression(param.defaultValue)));
      }
      return new tree.VariableDefinitions(
          null,
          makeEmptyModifiers(),
          singleton(definition));
    } else {
      tree.Node definition;
      if (param.defaultValue != null) {
        definition = new tree.SendSet(
            null,
            makeIdentifier(param.name),
            new tree.Operator(assignOperator),
            singleton(makeExpression(param.defaultValue)));
      } else {
        definition = makeIdentifier(param.name);
      }
      if (param.element != null) {
        setElement(definition, param.element, param);
      }
      return new tree.VariableDefinitions(
          param.type == null ? null : makeType(param.type),
          makeEmptyModifiers(), // TODO: Parameter modifiers?
          singleton(definition));
    }
  }

  tree.Modifiers makeEmptyModifiers() {
    return new tree.Modifiers(blankList());
  }

  tree.Modifiers makeModifiers({bool isExternal: false,
                                bool isStatic: false,
                                bool isAbstract: false,
                                bool isFactory: false,
                                bool isConst: false,
                                bool isFinal: false,
                                bool isVar: false}) {
    List<tree.Node> nodes = [];
    if (isExternal) {
      nodes.add(makeIdentifier('external'));
    }
    if (isStatic) {
      nodes.add(makeIdentifier('static'));
    }
    if (isAbstract) {
      nodes.add(makeIdentifier('abstract'));
    }
    if (isFactory) {
      nodes.add(makeIdentifier('factory'));
    }
    if (isConst) {
      nodes.add(makeIdentifier('const'));
    }
    if (isFinal) {
      nodes.add(makeIdentifier('final'));
    }
    if (isVar) {
      nodes.add(makeIdentifier('var'));
    }
    return new tree.Modifiers(makeList(' ', nodes));
  }

  tree.Modifiers makeVarModifiers({bool isConst: false,
                                   bool isFinal: false,
                                   bool useVar: false,
                                   bool isStatic: false}) {
    return makeModifiers(isStatic: isStatic,
                         isConst: isConst,
                         isFinal: isFinal,
                         isVar: useVar && !(isConst || isFinal));
  }

  tree.Modifiers makeFunctionModifiers(FunctionExpression exp) {
    if (exp.element == null) return makeEmptyModifiers();
    return makeModifiers(isExternal: exp.element.isExternal,
                         isStatic: exp.element.isStatic,
                         isFactory: exp.element.isFactoryConstructor,
                         isConst: exp.element.isConst);
  }

  tree.Node makeNodeForClassElement(elements.ClassElement cls) {
    if (cls.isMixinApplication) {
      return makeNamedMixinApplication(cls);
    } else if (cls.isEnumClass) {
      return makeEnum(cls);
    } else {
      return makeClassNode(cls);
    }
  }

  tree.Typedef makeTypedef(elements.TypedefElement typdef) {
    types.FunctionType functionType = typdef.alias;
    final tree.TypeAnnotation returnType =
        makeType(TypeGenerator.createType(functionType.returnType));

    final tree.Identifier name = makeIdentifier(typdef.name);
    final tree.NodeList typeParameters =
        makeTypeParameters(typdef.typeVariables);
    final tree.NodeList formals =
        makeParameters(TypeGenerator.createParametersFromType(functionType));

    final Token typedefKeyword = typedefToken;
    final Token endToken = semicolon;

    return new tree.Typedef(returnType, name, typeParameters, formals,
          typedefKeyword, endToken);
  }

  /// Create a [tree.NodeList] containing the type variable declarations in
  /// [typeVaraiables.
  tree.NodeList makeTypeParameters(List<types.DartType> typeVariables) {
    if (typeVariables.isEmpty) {
      return new tree.NodeList.empty();
    } else {
      List<tree.Node> typeVariableList = <tree.Node>[];
      for (types.TypeVariableType typeVariable in typeVariables) {
        tree.Node id = makeIdentifier(typeVariable.name);
        treeElements[id] = typeVariable.element;
        tree.Node bound;
        if (!typeVariable.element.bound.isObject) {
          bound =
              makeType(TypeGenerator.createType(typeVariable.element.bound));
        }
        tree.TypeVariable node = new tree.TypeVariable(id, bound);
        treeElements.setType(node, typeVariable);
        typeVariableList.add(node);
      }
      return makeList(',', typeVariableList, open: lt, close: gt);
    }
  }

  /// Create a [tree.NodeList] containing the declared interfaces.
  ///
  /// [interfaces] is from [elements.ClassElement] in reverse declaration order
  /// and it contains mixins. To produce a list of the declared interfaces only,
  /// interfaces in [mixinTypes] are omitted.
  ///
  /// [forNamedMixinApplication] is because the structure of the [tree.NodeList]
  /// differs between [tree.NamedMixinApplication] and [tree.ClassNode].
  // TODO(johnniwinther): Normalize interfaces on[tree.NamedMixinApplication]
  // and [tree.ClassNode].
  tree.NodeList makeInterfaces(Link<types.DartType> interfaces,
                               Set<types.DartType> mixinTypes,
                               {bool forNamedMixinApplication: false}) {
    Link<tree.Node> typeAnnotations = const Link<tree.Node>();
    for (Link<types.DartType> link = interfaces;
         !link.isEmpty;
         link = link.tail) {
      types.DartType interface = link.head;
      if (!mixinTypes.contains(interface)) {
        typeAnnotations = typeAnnotations.prepend(
            makeType(TypeGenerator.createType(interface)));
      }
    }
    if (typeAnnotations.isEmpty) {
      return forNamedMixinApplication ? null : new tree.NodeList.empty();
    } else {
      return new tree.NodeList(
          forNamedMixinApplication ? null : implementsToken,
          typeAnnotations, null, ',');
    }
  }

  /// Creates a [tree.NamedMixinApplication] node for [cls].
  // TODO(johnniwinther): Unify creation of mixin lists between
  // [NamedMixinApplicationElement] and [ClassElement].
  tree.NamedMixinApplication makeNamedMixinApplication(
       elements.MixinApplicationElement cls) {

    assert(invariant(cls, !cls.isUnnamedMixinApplication,
        message: "Cannot create ClassNode for unnamed mixin application "
                 "$cls."));
    tree.Modifiers modifiers = makeModifiers(isAbstract: cls.isAbstract);
    tree.Identifier name = makeIdentifier(cls.name);
    tree.NodeList typeParameters = makeTypeParameters(cls.typeVariables);

    Set<types.DartType> mixinTypes = new Set<types.DartType>();
    Link<tree.Node> mixins = const Link<tree.Node>();

    void addMixin(types.DartType mixinType) {
      mixinTypes.add(mixinType);
      mixins = mixins.prepend(makeType(TypeGenerator.createType(mixinType)));
    }

    addMixin(cls.mixinType);

    tree.Node superclass;
    types.InterfaceType supertype = cls.supertype;
    while (supertype.element.isUnnamedMixinApplication) {
      elements.MixinApplicationElement mixinApplication = supertype.element;
      addMixin(cls.asInstanceOf(mixinApplication.mixin));
      supertype = mixinApplication.supertype;
    }
    superclass =
        makeType(TypeGenerator.createType(cls.asInstanceOf(supertype.element)));
    tree.Node supernode = new tree.MixinApplication(
        superclass, new tree.NodeList(null, mixins, null, ','));

    tree.NodeList interfaces = makeInterfaces(
        cls.interfaces, mixinTypes, forNamedMixinApplication: true);

    return new tree.NamedMixinApplication(
        name, typeParameters, modifiers, supernode,
        interfaces, classToken, semicolon);
  }

  tree.Enum makeEnum(elements.EnumClassElement cls) {
    return new tree.Enum(
        enumToken,
        makeIdentifier(cls.name),
        makeList(',', cls.enumValues.map((e) => makeIdentifier(e.name)),
                 open: openBrace, close: closeBrace));
  }

  /// Creates a [tree.ClassNode] node for [cls].
  tree.ClassNode makeClassNode(elements.ClassElement cls) {
    assert(invariant(cls, !cls.isUnnamedMixinApplication,
        message: "Cannot create ClassNode for unnamed mixin application "
                 "$cls."));
    tree.Modifiers modifiers = makeModifiers(isAbstract: cls.isAbstract);
    tree.Identifier name = makeIdentifier(cls.name);
    tree.NodeList typeParameters = makeTypeParameters(cls.typeVariables);
    tree.Node supernode;
    types.InterfaceType supertype = cls.supertype;
    Set<types.DartType> mixinTypes = new Set<types.DartType>();
    Link<tree.Node> mixins = const Link<tree.Node>();

    void addMixin(types.DartType mixinType) {
      mixinTypes.add(mixinType);
      mixins = mixins.prepend(makeType(TypeGenerator.createType(mixinType)));
    }

    if (supertype != null) {
      if (supertype.element.isUnnamedMixinApplication) {
        while (supertype.element.isUnnamedMixinApplication) {
          elements.MixinApplicationElement mixinApplication = supertype.element;
          addMixin(cls.asInstanceOf(mixinApplication.mixin));
          supertype = mixinApplication.supertype;
        }
        tree.Node superclass = makeType(
            TypeGenerator.createType(cls.asInstanceOf(supertype.element)));
        supernode = new tree.MixinApplication(
            superclass, new tree.NodeList(null, mixins, null, ','));
      } else if (!supertype.isObject) {
        supernode = makeType(TypeGenerator.createType(supertype));
      }
    }
    tree.NodeList interfaces = makeInterfaces(
        cls.interfaces, mixinTypes);

    Token extendsKeyword = supernode != null ? extendsToken : null;
    return new tree.ClassNode(
        modifiers, name, typeParameters, supernode,
        interfaces, openBrace, extendsKeyword,
        null, // No body.
        closeBrace);
  }

  tree.Node constructorName(ConstructorDefinition exp) {
    String name = exp.name;
    tree.Identifier className = makeIdentifier(exp.element.enclosingClass.name);
    tree.Node result = name == ""
        ? className
        : new tree.Send(className, makeIdentifier(name));
    setElement(result, exp.element, exp);
    return result;
  }

  tree.Node functionName(FunctionExpression exp) {
    String name = exp.name;
    if (name == null) return null;
    if (isUserDefinableOperator(name)) {
      return makeOperator("operator$name");
    } else if (name == "unary-") {
      return makeOperator("operator-");
    }
    return makeIdentifier(name);
  }

  tree.Node parenthesize(tree.Node node) {
    return new tree.ParenthesizedExpression(node, openParen);
  }

  tree.Node unparseStringLiteral(Expression exp) {
    StringLiteralOutput output = Unparser.analyzeStringLiteral(exp);
    List parts = output.parts;
    tree.Node printStringChunk(StringChunk chunk) {
      bool raw = chunk.quoting.raw;
      int quoteCode = chunk.quoting.quote;

      List<tree.StringInterpolationPart> literalParts = [];
      tree.LiteralString firstLiteral;
      tree.Node currentInterpolation;

      // sb contains the current unfinished LiteralString
      StringBuffer sb = new StringBuffer();
      if (raw) {
        sb.write('r');
      }
      for (int i = 0; i < chunk.quoting.leftQuoteCharCount; i++) {
        sb.write(chunk.quoting.quoteChar);
      }

      // Print every character and string interpolation
      int startIndex = chunk.previous != null ? chunk.previous.endIndex : 0;
      for (int i = startIndex; i < chunk.endIndex; i++) {
        var part = parts[i];
        if (part is Expression) {
          // Finish the previous string interpolation, if there is one.
          tree.LiteralString lit = makeVerbatimStringLiteral(sb.toString());
          if (currentInterpolation != null) {
            literalParts.add(new tree.StringInterpolationPart(
                currentInterpolation,
                lit));
          } else {
            firstLiteral = lit;
          }
          sb.clear();
          currentInterpolation = makeExpression(part);
        } else {
          int char = part;
          sb.write(Unparser.getEscapedCharacter(char, quoteCode, raw));
        }
      }

      // Print ending quotes
      for (int i = 0; i < chunk.quoting.rightQuoteLength; i++) {
        sb.write(chunk.quoting.quoteChar);
      }

      // Finish the previous string interpolation, if there is one.
      // Then wrap everything in a StringInterpolation, if relevant.
      tree.LiteralString lit = makeVerbatimStringLiteral(sb.toString());
      tree.Node node;
      if (firstLiteral == null) {
        node = lit;
      } else {
        literalParts.add(new tree.StringInterpolationPart(
            currentInterpolation,
            lit));
        node = new tree.StringInterpolation(
            firstLiteral,
            makeList('', literalParts));
      }

      // Juxtapose with the previous string chunks, if any.
      if (chunk.previous != null) {
        return new tree.StringJuxtaposition(
            printStringChunk(chunk.previous),
            node);
      } else {
        return node;
      }
    }
    return printStringChunk(output.chunk);
  }

}