diff --git a/lib/parser.dart b/lib/parser.dart
index 92f1652..840b9f2 100644
--- a/lib/parser.dart
+++ b/lib/parser.dart
@@ -30,7 +30,7 @@
       : super(tokenizer);
 }
 
-void _createMessages({List errors, List options}) {
+void _createMessages({List<Message> errors, List<String> options}) {
   if (errors == null) errors = [];
 
   if (options == null) {
@@ -45,8 +45,11 @@
 
 // TODO(terry): Remove nested name parameter.
 /** Parse and analyze the CSS file. */
-StyleSheet compile(var input, {List errors, List options, bool nested: true,
-    bool polyfill: false, List<StyleSheet> includes: null}) {
+StyleSheet compile(var input, {List<Message> errors, List<String> options,
+    bool nested: true,
+    bool polyfill: false,
+    List<StyleSheet> includes: null}) {
+
   if (includes == null) {
     includes = [];
   }
@@ -70,7 +73,9 @@
 }
 
 /** Analyze the CSS file. */
-void analyze(List<StyleSheet> styleSheets,  {List errors, List options}) {
+void analyze(List<StyleSheet> styleSheets,
+    {List<Message> errors, List<String> options}) {
+
   _createMessages(errors: errors, options: options);
   new Analyzer(styleSheets, messages).run();
 }
@@ -80,7 +85,7 @@
  * or [List<int>] of bytes and returns a [StyleSheet] AST.  The optional
  * [errors] list will contain each error/warning as a [Message].
  */
-StyleSheet parse(var input, {List errors, List options}) {
+StyleSheet parse(var input, {List<Message> errors, List<String> options}) {
   var source = _inputAsString(input);
 
   _createMessages(errors: errors, options: options);
@@ -95,7 +100,7 @@
  * or [List<int>] of bytes and returns a [StyleSheet] AST.  The optional
  * [errors] list will contain each error/warning as a [Message].
  */
-StyleSheet selector(var input, {List errors}) {
+StyleSheet selector(var input, {List<Message> errors}) {
   var source = _inputAsString(input);
 
   _createMessages(errors: errors);
@@ -150,7 +155,7 @@
 
 /** A simple recursive descent parser for CSS. */
 class _Parser {
-  Tokenizer tokenizer;
+  final Tokenizer tokenizer;
 
   /** Base url of CSS file. */
   final String _baseUrl;
@@ -1184,9 +1189,9 @@
   /**
    * Return list of selectors
    */
-  processSelector() {
-    List<SimpleSelectorSequence> simpleSequences = [];
-    int start = _peekToken.start;
+  Selector processSelector() {
+    var simpleSequences = <SimpleSelectorSequence>[];
+    var start = _peekToken.start;
     while (true) {
       // First item is never descendant make sure it's COMBINATOR_NONE.
       var selectorItem = simpleSelectorSequence(simpleSequences.length == 0);
@@ -1344,12 +1349,12 @@
    */
   simpleSelectorTail() {
     // Check for HASH | class | attrib | pseudo | negation
-    int start = _peekToken.start;
+    var start = _peekToken.start;
     switch (_peek()) {
       case TokenKind.HASH:
         _eat(TokenKind.HASH);
 
-        bool hasWhiteSpace = false;
+        var hasWhiteSpace = false;
         if (_anyWhiteSpaceBeforePeekToken(TokenKind.HASH)) {
           _warning("Not a valid ID selector expected #id", _makeSpan(start));
           hasWhiteSpace = true;
@@ -1395,7 +1400,7 @@
     // :pseudo-class ::pseudo-element
     // TODO(terry): '::' should be token.
     _eat(TokenKind.COLON);
-    bool pseudoElement = _maybeEat(TokenKind.COLON);
+    var pseudoElement = _maybeEat(TokenKind.COLON);
 
     // TODO(terry): If no identifier specified consider optimizing out the
     //              : or :: and making this a normal selector.  For now,
@@ -1456,7 +1461,7 @@
    *    NUMBER            {num}
    */
   processSelectorExpression() {
-    int start = _peekToken.start;
+    var start = _peekToken.start;
 
     var expression = new SelectorExpression(_makeSpan(start));
 
@@ -1467,7 +1472,7 @@
     // operator not identifier.
     tokenizer.selectorExpression = true;
 
-    bool keepParsing = true;
+    var keepParsing = true;
     while (keepParsing) {
       switch (_peek()) {
         case TokenKind.PLUS:
@@ -1539,8 +1544,8 @@
   //  SUBSTRMATCH:      '*='
   //
   //
-  processAttribute() {
-    int start = _peekToken.start;
+  AttributeSelector processAttribute() {
+    var start = _peekToken.start;
 
     if (_maybeEat(TokenKind.LBRACK)) {
       var attrName = identifier();
@@ -1593,7 +1598,7 @@
   //   *IDENT                   - IE7 or below
   //   _IDENT                   - IE6 property (automatically a valid ident)
   //
-  processDeclaration(List dartStyles) {
+  Declaration processDeclaration(List dartStyles) {
     Declaration decl;
 
     int start = _peekToken.start;
@@ -1637,7 +1642,7 @@
       var include = processInclude(span, eatSemiColon: false);
       decl = new IncludeMixinAtDeclaration(include, span);
     } else if (_peekToken.kind == TokenKind.DIRECTIVE_EXTEND) {
-      List<SimpleSelectorSequence> simpleSequences = [];
+      var simpleSequences = <TreeNode>[];
 
       _next();
       var span = _makeSpan(start);
@@ -1730,14 +1735,11 @@
     'normal' : FontWeight.normal
   };
 
-  static _findStyle(String styleName) {
-    if (_stylesToDart.containsKey(styleName)) {
-      return _stylesToDart[styleName];
-    }
-  }
+  static int _findStyle(String styleName) => _stylesToDart[styleName];
 
-  _styleForDart(Identifier property, Expressions exprs, List dartStyles) {
-    int styleType = _findStyle(property.name.toLowerCase());
+  DartStyleExpression _styleForDart(Identifier property, Expressions exprs,
+      List dartStyles) {
+    var styleType = _findStyle(property.name.toLowerCase());
     if (styleType != null) {
       return buildDartStyleNode(styleType, exprs, dartStyles);
     }
@@ -1754,7 +1756,9 @@
     return fontExpr;
   }
 
-  buildDartStyleNode(int styleType, Expressions exprs, List dartStyles) {
+  DartStyleExpression buildDartStyleNode(int styleType, Expressions exprs,
+      List dartStyles) {
+
     switch (styleType) {
       /*
        * Properties in order:
@@ -1897,7 +1901,7 @@
 
   // TODO(terry): Look at handling width of thin, thick, etc. any none numbers
   //              to convert to a number.
-  processOneNumber(Expressions exprs, int part) {
+  DartStyleExpression processOneNumber(Expressions exprs, int part) {
     var value = marginValue(exprs.expressions[0]);
     if (value != null) {
       switch (part) {
@@ -1947,7 +1951,7 @@
    *
    * The values of the margins can be a unit or unitless or auto.
    */
-  processFourNums(Expressions exprs) {
+  BoxEdge processFourNums(Expressions exprs) {
     num top;
     num right;
     num bottom;
@@ -2000,16 +2004,16 @@
   //  operator:     '/' | ','
   //  term:         (see processTerm)
   //
-  processExpr([bool ieFilter = false]) {
-    int start = _peekToken.start;
-    Expressions expressions = new Expressions(_makeSpan(start));
+  Expressions processExpr([bool ieFilter = false]) {
+    var start = _peekToken.start;
+    var expressions = new Expressions(_makeSpan(start));
 
-    bool keepGoing = true;
+    var keepGoing = true;
     var expr;
     while (keepGoing && (expr = processTerm(ieFilter)) != null) {
       var op;
 
-      int opStart = _peekToken.start;
+      var opStart = _peekToken.start;
 
       switch (_peek()) {
       case TokenKind.SLASH:
@@ -2061,7 +2065,7 @@
     return expressions;
   }
 
-  static int MAX_UNICODE = int.parse('0x10FFFF');
+  static final int MAX_UNICODE = int.parse('0x10FFFF');
 
   //  Term grammar:
   //
@@ -2085,7 +2089,7 @@
   //  function:     IDENT '(' expr ')'
   //
   processTerm([bool ieFilter = false]) {
-    int start = _peekToken.start;
+    var start = _peekToken.start;
     Token t;                          // token for term's value
     var value;                        // value of term (numeric values)
 
@@ -2193,8 +2197,8 @@
       }
 
       // Yes, process the color as an RGB value.
-      String rgbColor = TokenKind.decimalToHex(
-          TokenKind.colorValue(colorEntry), 6);
+      var rgbColor =
+          TokenKind.decimalToHex(TokenKind.colorValue(colorEntry), 6);
       return _parseHex(rgbColor, _makeSpan(start));
     case TokenKind.UNICODE_RANGE:
       var first;
@@ -2248,8 +2252,8 @@
   }
 
   /** Process all dimension units. */
-  processDimension(Token t, var value, Span span) {
-    var term;
+  LiteralTerm processDimension(Token t, var value, Span span) {
+    LiteralTerm term;
     var unitType = this._peek();
 
     switch (unitType) {
@@ -2328,11 +2332,11 @@
     return term;
   }
 
-  processQuotedString([bool urlString = false]) {
-    int start = _peekToken.start;
+  String processQuotedString([bool urlString = false]) {
+    var start = _peekToken.start;
 
     // URI term sucks up everything inside of quotes(' or ") or between parens
-    int stopToken = urlString ? TokenKind.RPAREN : -1;
+    var stopToken = urlString ? TokenKind.RPAREN : -1;
     switch (_peek()) {
     case TokenKind.SINGLE_QUOTE:
       stopToken = TokenKind.SINGLE_QUOTE;
@@ -2358,13 +2362,13 @@
     }
 
     // Gobble up everything until we hit our stop token.
-    int runningStart = _peekToken.start;
+    var runningStart = _peekToken.start;
     while (_peek() != stopToken && _peek() != TokenKind.END_OF_FILE) {
       var tok = _next();
     }
 
     // All characters between quotes is the string.
-    int end = _peekToken.end;
+    var end = _peekToken.end;
     var stringValue = (_peekToken.span as FileSpan).file.getText(start,
         end - 1);
 
@@ -2386,7 +2390,7 @@
    * then parse to the right paren ignoring everything in between.
    */
   processIEFilter(int startAfterProgidColon) {
-    int parens = 0;
+    var parens = 0;
 
     while (_peek() != TokenKind.END_OF_FILE) {
       switch (_peek()) {
@@ -2413,14 +2417,14 @@
   //  function:     IDENT '(' expr ')'
   //
   processFunction(Identifier func) {
-    int start = _peekToken.start;
+    var start = _peekToken.start;
 
-    String name = func.name;
+    var name = func.name;
 
     switch (name) {
     case 'url':
       // URI term sucks up everything inside of quotes(' or ") or between parens
-      String urlParam = processQuotedString(true);
+      var urlParam = processQuotedString(true);
 
       // TODO(terry): Better error messge and checking for mismatched quotes.
       if (_peek() == TokenKind.END_OF_FILE) {
@@ -2470,7 +2474,7 @@
     return null;
   }
 
-  identifier() {
+  Identifier identifier() {
     var tok = _next();
 
     if (!TokenKind.isIdentifier(tok.kind) &&
@@ -2486,7 +2490,7 @@
 
   // TODO(terry): Move this to base <= 36 and into shared code.
   static int _hexDigit(int c) {
-    if(c >= 48/*0*/ && c <= 57/*9*/) {
+    if (c >= 48/*0*/ && c <= 57/*9*/) {
       return c - 48;
     } else if (c >= 97/*a*/ && c <= 102/*f*/) {
       return c - 87;
@@ -2498,9 +2502,9 @@
   }
 
   HexColorTerm _parseHex(String hexText, Span span) {
-    int hexValue = 0;
+    var hexValue = 0;
 
-     for (int i = 0; i < hexText.length; i++) {
+     for (var i = 0; i < hexText.length; i++) {
       var digit = _hexDigit(hexText.codeUnitAt(i));
       if (digit < 0) {
         _warning('Bad hex number', span);
@@ -2535,7 +2539,7 @@
   ExpressionsProcessor(this._exprs);
 
   // TODO(terry): Only handles ##px unit.
-  processFontSize() {
+  FontExpression processFontSize() {
     /* font-size[/line-height]
      *
      * Possible size values:
@@ -2553,7 +2557,7 @@
      */
     LengthTerm size;
     LineHeight lineHt;
-    bool nextIsLineHeight = false;
+    var nextIsLineHeight = false;
     for (; _index < _exprs.expressions.length; _index++) {
       var expr = _exprs.expressions[_index];
       if (size == null && expr is LengthTerm) {
@@ -2580,14 +2584,14 @@
     return new FontExpression(_exprs.span, size: size, lineHeight: lineHt);
   }
 
-  processFontFamily() {
-    final List<String> family = <String>[];
+  FontExpression processFontFamily() {
+    var family = <String>[];
 
     /* Possible family values:
      * font-family: arial, Times new roman ,Lucida Sans Unicode,Courier;
      * font-family: "Times New Roman", arial, Lucida Sans Unicode, Courier;
      */
-    bool moreFamilies = false;
+    var moreFamilies = false;
 
     for (; _index < _exprs.expressions.length; _index++) {
       Expression expr = _exprs.expressions[_index];
@@ -2609,8 +2613,8 @@
     return new FontExpression(_exprs.span, family: family);
   }
 
-  processFont() {
-    var family;
+  FontExpression processFont() {
+    List<String> family;
 
     // Process all parts of the font expression.
     FontExpression fontSize;
@@ -2645,8 +2649,8 @@
   StringBuffer result = null;
 
   for (int i = 0; i < text.length; i++) {
-    int code = text.codeUnitAt(i);
-    var replace = null;
+    var code = text.codeUnitAt(i);
+    String replace = null;
     switch (code) {
       case 34/*'"'*/:  if (!single) replace = r'\"'; break;
       case 39/*"'"*/:  if (single) replace = r"\'"; break;
diff --git a/lib/src/analyzer.dart b/lib/src/analyzer.dart
index cd0c6fb..e0ec078 100644
--- a/lib/src/analyzer.dart
+++ b/lib/src/analyzer.dart
@@ -573,7 +573,7 @@
  * parameters.
  */
 class CallMixin extends Visitor {
-  var mixinDef;
+  final MixinDefinition mixinDef;
   List _definedArgs;
   Expressions _currExpressions;
   int _currIndex = -1;
@@ -595,7 +595,7 @@
    * Given a mixin's defined arguments return a cloned mixin defintion that has
    * replaced all defined arguments with user's supplied VarUsages.
    */
-  transform(List<TreeNode> callArgs) {
+  MixinDefinition transform(List<TreeNode> callArgs) {
     // TODO(terry): Handle default arguments and varArgs.
     // Transform mixin with callArgs.
     var index = 0;
@@ -620,7 +620,6 @@
       }
 
       var expressions = varUsages[varDef.definedName];
-      var expressionsLength = expressions.length;
       expressions.forEach((k, v) {
         for (var usagesIndex in v) {
           k.expressions.replaceRange(usagesIndex, usagesIndex + 1, callArg);
@@ -724,7 +723,7 @@
   bool _allIncludes(rulesets) =>
       rulesets.every((rule) => rule is IncludeDirective || rule is NoOp);
 
-  CallMixin _createCallDeclMixin(mixinDef) {
+  CallMixin _createCallDeclMixin(MixinDefinition mixinDef) {
     callMap.putIfAbsent(mixinDef.name, () =>
         callMap[mixinDef.name] = new CallMixin(mixinDef, varDefs));
     return callMap[mixinDef.name];
@@ -968,8 +967,8 @@
  * Changes any selector that matches @extend.
  */
 class InheritExtends extends Visitor {
-  Messages _messages;
-  AllExtends _allExtends;
+  final Messages _messages;
+  final AllExtends _allExtends;
 
   InheritExtends(this._messages, this._allExtends);
 
diff --git a/lib/src/css_printer.dart b/lib/src/css_printer.dart
index 73c2984..9a80ee1 100644
--- a/lib/src/css_printer.dart
+++ b/lib/src/css_printer.dart
@@ -104,7 +104,7 @@
     emit(' ');
 
     var declsMargin = node._declsMargin;
-    int declsMarginLength = declsMargin.length;
+    var declsMarginLength = declsMargin.length;
     for (var i = 0; i < declsMarginLength; i++) {
       if (i > 0) emit(_newLine);
       emit('{$_newLine');
@@ -161,7 +161,7 @@
   }
 
   void visitStyletDirective(StyletDirective node) {
-    emit('/* @stylet export as ${node._dartClassName} */\n');
+    emit('/* @stylet export as ${node.dartClassName} */\n');
   }
 
   void visitNamespaceDirective(NamespaceDirective node) {
@@ -224,7 +224,7 @@
   }
 
   void visitDeclarationGroup(DeclarationGroup node) {
-    var declarations = node._declarations;
+    var declarations = node.declarations;
     var declarationsLength = declarations.length;
     for (var i = 0; i < declarationsLength; i++) {
       if (i > 0) emit(_newLine);
@@ -338,7 +338,7 @@
   }
 
   void visitSelectorExpression(SelectorExpression node) {
-    var expressions = node._expressions;
+    var expressions = node.expressions;
     var expressionsLength = expressions.length;
     for (var i = 0; i < expressionsLength; i++) {
       // Add space seperator between terms without an operator.
diff --git a/lib/src/messages.dart b/lib/src/messages.dart
index 829b419..24964ad 100644
--- a/lib/src/messages.dart
+++ b/lib/src/messages.dart
@@ -68,7 +68,7 @@
   }
 }
 
-typedef void PrintHandler(Object obj);
+typedef void PrintHandler(Message obj);
 
 /**
  * This class tracks and prints information, warnings, and errors emitted by the
diff --git a/lib/src/tree.dart b/lib/src/tree.dart
index 941eb57..f3c71eb 100644
--- a/lib/src/tree.dart
+++ b/lib/src/tree.dart
@@ -23,19 +23,25 @@
 class Wildcard extends TreeNode {
   Wildcard(Span span): super(span);
   Wildcard clone() => new Wildcard(span);
-  visit(VisitorBase visitor) => visitor.visitWildcard(this);
+  void visit(VisitorBase visitor) => visitor.visitWildcard(this);
+
+  String get name => '*';
 }
 
 class ThisOperator extends TreeNode {
   ThisOperator(Span span): super(span);
   ThisOperator clone() => new ThisOperator(span);
-  visit(VisitorBase visitor) => visitor.visitThisOperator(this);
+  void visit(VisitorBase visitor) => visitor.visitThisOperator(this);
+
+  String get name => '&';
 }
 
 class Negation extends TreeNode {
   Negation(Span span): super(span);
   Negation clone() => new Negation(span);
-  visit(VisitorBase visitor) => visitor.visitNegation(this);
+  void visit(VisitorBase visitor) => visitor.visitNegation(this);
+
+  String get name => 'not';
 }
 
 // /*  ....   */
@@ -44,14 +50,14 @@
 
   CssComment(this.comment, Span span): super(span);
   CssComment clone() => new CssComment(comment, span);
-  visit(VisitorBase visitor) => visitor.visitCssComment(this);
+  void visit(VisitorBase visitor) => visitor.visitCssComment(this);
 }
 
 // CDO/CDC (Comment Definition Open <!-- and Comment Definition Close -->).
 class CommentDefinition extends CssComment {
   CommentDefinition(String comment, Span span): super(comment, span);
   CommentDefinition clone() => new CommentDefinition(comment, span);
-  visit(VisitorBase visitor) => visitor.visitCommentDefinition(this);
+  void visit(VisitorBase visitor) => visitor.visitCommentDefinition(this);
 }
 
 class SelectorGroup extends TreeNode {
@@ -110,32 +116,33 @@
       new SimpleSelectorSequence(simpleSelector, span, combinator);
 
   void visit(VisitorBase visitor) => visitor.visitSimpleSelectorSequence(this);
+
+  String toString() => simpleSelector.name;
 }
 
 /* All other selectors (element, #id, .class, attribute, pseudo, negation,
  * namespace, *) are derived from this selector.
  */
-class SimpleSelector extends TreeNode {
-  final _name; // Wildcard, ThisOperator, Identifier, others?
+abstract class SimpleSelector extends TreeNode {
+  final _name; // Wildcard, ThisOperator, Identifier, Negation, others?
 
   SimpleSelector(this._name, Span span) : super(span);
 
-  // Name can be an Identifier or WildCard we'll return either the name or '*'.
-  String get name => isWildcard ? '*' : isThis ? '&' : _name.name;
+  String get name => _name.name;
 
   bool get isWildcard => _name is Wildcard;
 
   bool get isThis => _name is ThisOperator;
 
-  SimpleSelector clone() => new SimpleSelector(_name, span);
-
   void visit(VisitorBase visitor) => visitor.visitSimpleSelector(this);
 }
 
 // element name
 class ElementSelector extends SimpleSelector {
   ElementSelector(name, Span span) : super(name, span);
-  visit(VisitorBase visitor) => visitor.visitElementSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitElementSelector(this);
+
+  ElementSelector clone() => new ElementSelector(_name, span);
 
   String toString() => name;
 }
@@ -155,7 +162,7 @@
 
   NamespaceSelector clone() => new NamespaceSelector(_namespace, "", span);
 
-  visit(VisitorBase visitor) => visitor.visitNamespaceSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitNamespaceSelector(this);
 
   String toString() => "$namespace|${nameAsSimpleSelector.name}";
 }
@@ -219,7 +226,7 @@
 
   AttributeSelector clone() => new AttributeSelector(_name, _op, _value, span);
 
-  visit(VisitorBase visitor) => visitor.visitAttributeSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitAttributeSelector(this);
 
   String toString() => "[$name${matchOperator()}${valueToString()}]";
 }
@@ -228,7 +235,7 @@
 class IdSelector extends SimpleSelector {
   IdSelector(Identifier name, Span span) : super(name, span);
   IdSelector clone() => new IdSelector(_name, span);
-  visit(VisitorBase visitor) => visitor.visitIdSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitIdSelector(this);
 
   String toString() => "#$_name";
 }
@@ -237,7 +244,7 @@
 class ClassSelector extends SimpleSelector {
   ClassSelector(Identifier name, Span span) : super(name, span);
   ClassSelector clone() => new ClassSelector(_name, span);
-  visit(VisitorBase visitor) => visitor.visitClassSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitClassSelector(this);
 
   String toString() => ".$_name";
 }
@@ -245,7 +252,9 @@
 // :pseudoClass
 class PseudoClassSelector extends SimpleSelector {
   PseudoClassSelector(Identifier name, Span span) : super(name, span);
-  visit(VisitorBase visitor) => visitor.visitPseudoClassSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitPseudoClassSelector(this);
+
+  PseudoClassSelector clone() => new PseudoClassSelector(_name, span);
 
   String toString() => ":$name";
 }
@@ -253,59 +262,64 @@
 // ::pseudoElement
 class PseudoElementSelector extends SimpleSelector {
   PseudoElementSelector(Identifier name, Span span) : super(name, span);
-  visit(VisitorBase visitor) => visitor.visitPseudoElementSelector(this);
+  void visit(VisitorBase visitor) => visitor.visitPseudoElementSelector(this);
+
+  PseudoElementSelector clone() => new PseudoElementSelector(_name, span);
 
   String toString() => "::$name";
 }
 
 // :pseudoClassFunction(expression)
 class PseudoClassFunctionSelector extends PseudoClassSelector {
-  SelectorExpression expression;
+  final SelectorExpression expression;
 
   PseudoClassFunctionSelector(Identifier name, this.expression, Span span)
       : super(name, span);
+
   PseudoClassFunctionSelector clone() =>
       new PseudoClassFunctionSelector(_name, expression, span);
-  visit(VisitorBase visitor) => visitor.visitPseudoClassFunctionSelector(this);
+
+  void visit(VisitorBase visitor) =>
+      visitor.visitPseudoClassFunctionSelector(this);
 }
 
 // ::pseudoElementFunction(expression)
 class PseudoElementFunctionSelector extends PseudoElementSelector {
-  SelectorExpression expression;
+  final SelectorExpression expression;
 
   PseudoElementFunctionSelector(Identifier name, this.expression, Span span)
       : super(name, span);
+
   PseudoElementFunctionSelector clone() =>
       new PseudoElementFunctionSelector(_name, expression, span);
-  visit(VisitorBase visitor) =>
+
+  void visit(VisitorBase visitor) =>
       visitor.visitPseudoElementFunctionSelector(this);
 }
 
 class SelectorExpression extends TreeNode {
-  final List<Expression> _expressions = [];
+  final List<Expression> expressions = [];
 
   SelectorExpression(Span span): super(span);
 
-  add(Expression expression) {
-    _expressions.add(expression);
+  void add(Expression expression) {
+    expressions.add(expression);
   }
 
-  List<Expression> get expressions => _expressions;
-
   SelectorExpression clone() {
     var selectorExpr = new SelectorExpression(span);
-    for (var expr in _expressions) {
+    for (var expr in expressions) {
       selectorExpr.add(expr.clone());
     }
     return selectorExpr;
   }
 
-  visit(VisitorBase visitor) => visitor.visitSelectorExpression(this);
+  void visit(VisitorBase visitor) => visitor.visitSelectorExpression(this);
 }
 
 // :NOT(negation_arg)
 class NegationSelector extends SimpleSelector {
-  SimpleSelector negationArg;
+  final SimpleSelector negationArg;
 
   NegationSelector(this.negationArg, Span span)
       : super(new Negation(span), span);
@@ -320,7 +334,7 @@
 
   NoOp clone() => new NoOp();
 
-  visit(VisitorBase visitor) => visitor.visitNoOp(this);
+  void visit(VisitorBase visitor) => visitor.visitNoOp(this);
 }
 
 class StyleSheet extends TreeNode {
@@ -349,7 +363,7 @@
 class TopLevelProduction extends TreeNode {
   TopLevelProduction(Span span) : super(span);
   TopLevelProduction clone() => new TopLevelProduction(span);
-  visit(VisitorBase visitor) => visitor.visitTopLevelProduction(this);
+  void visit(VisitorBase visitor) => visitor.visitTopLevelProduction(this);
 }
 
 class RuleSet extends TopLevelProduction {
@@ -367,7 +381,7 @@
     return new RuleSet(cloneSelectorGroup, cloneDeclarationGroup, span);
   }
 
-  visit(VisitorBase visitor) => visitor.visitRuleSet(this);
+  void visit(VisitorBase visitor) => visitor.visitRuleSet(this);
 }
 
 class Directive extends TreeNode {
@@ -456,7 +470,7 @@
     }
     return new MediaQuery(_mediaUnary, _mediaType, cloneExpressions, span);
   }
-  visit(VisitorBase visitor) => visitor.visitMediaQuery(this);
+  void visit(VisitorBase visitor) => visitor.visitMediaQuery(this);
 }
 
 class MediaDirective extends Directive {
@@ -585,24 +599,22 @@
 }
 
 class StyletDirective extends Directive {
-  final String _dartClassName;
-  final List<RuleSet> _rulesets;
+  final String dartClassName;
+  final List<RuleSet> rulesets;
 
-  StyletDirective(this._dartClassName, this._rulesets, Span span) : super(span);
+  StyletDirective(this.dartClassName, this.rulesets, Span span) : super(span);
 
   bool get isBuiltIn => false;
   bool get isExtension => true;
 
-  String get dartClassName => _dartClassName;
-  List<RuleSet> get rulesets => _rulesets;
-
   StyletDirective clone() {
     var cloneRulesets = [];
-    for (var ruleset in _rulesets) {
+    for (var ruleset in rulesets) {
       cloneRulesets.add(ruleset.clone());
     }
-    return new StyletDirective(_dartClassName, cloneRulesets, span);
+    return new StyletDirective(dartClassName, cloneRulesets, span);
   }
+
   void visit(VisitorBase visitor) => visitor.visitStyletDirective(this);
 }
 
@@ -616,6 +628,7 @@
   NamespaceDirective(this._prefix, this._uri, Span span) : super(span);
 
   NamespaceDirective clone() => new NamespaceDirective(_prefix, _uri, span);
+
   void visit(VisitorBase visitor) => visitor.visitNamespaceDirective(this);
 
   String get prefix => _prefix.length > 0 ? '$_prefix ' : '';
@@ -629,6 +642,7 @@
 
   VarDefinitionDirective clone() =>
       new VarDefinitionDirective(def.clone(), span);
+
   void visit(VisitorBase visitor) => visitor.visitVarDefinitionDirective(this);
 }
 
@@ -647,6 +661,7 @@
     }
     return new MixinDefinition(name, cloneDefinedArgs, varArgs, span);
   }
+
   void visit(VisitorBase visitor) => visitor.visitMixinDefinition(this);
 }
 
@@ -670,6 +685,7 @@
     return new MixinRulesetDirective(name, clonedArgs, varArgs, clonedRulesets,
         span);
   }
+
   void visit(VisitorBase visitor) => visitor.visitMixinRulesetDirective(this);
 }
 
@@ -679,6 +695,7 @@
   MixinDeclarationDirective(String name, List<VarDefinitionDirective>  args,
       bool varArgs, this.declarations, Span span) :
       super(name, args, varArgs, span);
+
   MixinDeclarationDirective clone() {
     var clonedArgs = [];
     for (var arg in definedArgs) {
@@ -687,6 +704,7 @@
     return new MixinDeclarationDirective(name, clonedArgs, varArgs,
         declarations.clone(), span);
   }
+
   void visit(VisitorBase visitor) => visitor.visitMixinDeclarationDirective(this);
 }
 
@@ -721,7 +739,7 @@
   final Identifier _property;
   final Expression _expression;
   /** Style exposed to Dart. */
-  dynamic _dart;
+  dynamic dartStyle;
   final bool important;
 
   /**
@@ -734,21 +752,17 @@
    */
   final bool isIE7;
 
-  Declaration(this._property, this._expression, this._dart, Span span,
+  Declaration(this._property, this._expression, this.dartStyle, Span span,
               {important: false, ie7: false})
       : this.important = important, this.isIE7 = ie7, super(span);
 
   String get property => isIE7 ? '*${_property.name}' : _property.name;
   Expression get expression => _expression;
 
-  bool get hasDartStyle => _dart != null;
-  get dartStyle => _dart;
-  set dartStyle(dStyle) {
-    _dart = dStyle;
-  }
+  bool get hasDartStyle => dartStyle != null;
 
   Declaration clone() =>
-      new Declaration(_property.clone(), _expression.clone(), _dart, span,
+      new Declaration(_property.clone(), _expression.clone(), dartStyle, span,
       important: important);
 
   void visit(VisitorBase visitor) => visitor.visitDeclaration(this);
@@ -768,11 +782,10 @@
 
   String get definedName => _property.name;
 
-  set dartStyle(dStyle) { }
-
   VarDefinition clone() =>
       new VarDefinition(_property.clone(),
       expression != null ? expression.clone() : null, span);
+
   void visit(VisitorBase visitor) => visitor.visitVarDefinition(this);
 }
 
@@ -797,16 +810,13 @@
 }
 
 class ExtendDeclaration extends Declaration {
-  List<SimpleSelectorSequence> selectors;
+  final List<TreeNode> selectors;
 
   ExtendDeclaration(this.selectors, Span span) :
       super(null, null, null, span);
 
   ExtendDeclaration clone() {
-    List<SimpleSelectorSequence> newSelector = [];
-    for (var selectorSeq in selectors) {
-      newSelector.add(selectorSeq.clone());
-    }
+    var newSelector = selectors.map((s) => s.clone()).toList();
     return new ExtendDeclaration(newSelector, span);
   }
 
@@ -815,17 +825,12 @@
 
 class DeclarationGroup extends TreeNode {
   /** Can be either Declaration or RuleSet (if nested selector). */
-  final List _declarations;
+  final List declarations;
 
-  DeclarationGroup(this._declarations, Span span) : super(span);
-
-  List get declarations => _declarations;
+  DeclarationGroup(this.declarations, Span span) : super(span);
 
   DeclarationGroup clone() {
-    var clonedDecls = [];
-    for (var decl in _declarations) {
-      clonedDecls.add(decl.clone());
-    }
+    var clonedDecls = declarations.map((d) => d.clone()).toList();
     return new DeclarationGroup(clonedDecls, span);
   }
 
@@ -917,17 +922,15 @@
 }
 
 class UnitTerm extends LiteralTerm {
-  final int _unit;
+  final int unit;
 
-  UnitTerm(value, String t, Span span, this._unit) : super(value, t, span);
+  UnitTerm(value, String t, Span span, this.unit) : super(value, t, span);
 
-  int get unit => _unit;
-
-  UnitTerm clone() => new UnitTerm(value, text, span, _unit);
+  UnitTerm clone() => new UnitTerm(value, text, span, unit);
 
   void visit(VisitorBase visitor) => visitor.visitUnitTerm(this);
 
-  String unitToString() => TokenKind.unitToString(_unit);
+  String unitToString() => TokenKind.unitToString(unit);
 
   String toString() => '$text${unitToString()}';
 }
@@ -935,14 +938,14 @@
 class LengthTerm extends UnitTerm {
   LengthTerm(value, String t, Span span,
       [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(this._unit == TokenKind.UNIT_LENGTH_PX ||
-        this._unit == TokenKind.UNIT_LENGTH_CM ||
-        this._unit == TokenKind.UNIT_LENGTH_MM ||
-        this._unit == TokenKind.UNIT_LENGTH_IN ||
-        this._unit == TokenKind.UNIT_LENGTH_PT ||
-        this._unit == TokenKind.UNIT_LENGTH_PC);
+    assert(this.unit == TokenKind.UNIT_LENGTH_PX ||
+        this.unit == TokenKind.UNIT_LENGTH_CM ||
+        this.unit == TokenKind.UNIT_LENGTH_MM ||
+        this.unit == TokenKind.UNIT_LENGTH_IN ||
+        this.unit == TokenKind.UNIT_LENGTH_PT ||
+        this.unit == TokenKind.UNIT_LENGTH_PC);
   }
-  LengthTerm clone() => new LengthTerm(value, text, span, _unit);
+  LengthTerm clone() => new LengthTerm(value, text, span, unit);
   void visit(VisitorBase visitor) => visitor.visitLengthTerm(this);
 }
 
@@ -967,10 +970,10 @@
 class AngleTerm extends UnitTerm {
   AngleTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(this._unit == TokenKind.UNIT_ANGLE_DEG ||
-        this._unit == TokenKind.UNIT_ANGLE_RAD ||
-        this._unit == TokenKind.UNIT_ANGLE_GRAD ||
-        this._unit == TokenKind.UNIT_ANGLE_TURN);
+    assert(this.unit == TokenKind.UNIT_ANGLE_DEG ||
+        this.unit == TokenKind.UNIT_ANGLE_RAD ||
+        this.unit == TokenKind.UNIT_ANGLE_GRAD ||
+        this.unit == TokenKind.UNIT_ANGLE_TURN);
   }
 
   AngleTerm clone() => new AngleTerm(value, text, span, unit);
@@ -980,9 +983,9 @@
 class TimeTerm extends UnitTerm {
   TimeTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(this._unit == TokenKind.UNIT_ANGLE_DEG ||
-        this._unit == TokenKind.UNIT_TIME_MS ||
-        this._unit == TokenKind.UNIT_TIME_S);
+    assert(this.unit == TokenKind.UNIT_ANGLE_DEG ||
+        this.unit == TokenKind.UNIT_TIME_MS ||
+        this.unit == TokenKind.UNIT_TIME_S);
   }
 
   TimeTerm clone() => new TimeTerm(value, text, span, unit);
@@ -992,7 +995,7 @@
 class FreqTerm extends UnitTerm {
   FreqTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(_unit == TokenKind.UNIT_FREQ_HZ || _unit == TokenKind.UNIT_FREQ_KHZ);
+    assert(unit == TokenKind.UNIT_FREQ_HZ || unit == TokenKind.UNIT_FREQ_KHZ);
   }
 
   FreqTerm clone() => new FreqTerm(value, text, span, unit);
@@ -1016,9 +1019,9 @@
 class ResolutionTerm extends UnitTerm {
   ResolutionTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(_unit == TokenKind.UNIT_RESOLUTION_DPI ||
-        _unit == TokenKind.UNIT_RESOLUTION_DPCM ||
-        _unit == TokenKind.UNIT_RESOLUTION_DPPX);
+    assert(unit == TokenKind.UNIT_RESOLUTION_DPI ||
+        unit == TokenKind.UNIT_RESOLUTION_DPCM ||
+        unit == TokenKind.UNIT_RESOLUTION_DPPX);
   }
 
   ResolutionTerm clone() => new ResolutionTerm(value, text, span, unit);
@@ -1028,7 +1031,7 @@
 class ChTerm extends UnitTerm {
   ChTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(_unit == TokenKind.UNIT_CH);
+    assert(unit == TokenKind.UNIT_CH);
   }
 
   ChTerm clone() => new ChTerm(value, text, span, unit);
@@ -1038,7 +1041,7 @@
 class RemTerm extends UnitTerm {
   RemTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(_unit == TokenKind.UNIT_REM);
+    assert(unit == TokenKind.UNIT_REM);
   }
 
   RemTerm clone() => new RemTerm(value, text, span, unit);
@@ -1048,10 +1051,10 @@
 class ViewportTerm extends UnitTerm {
   ViewportTerm(var value, String t, Span span,
     [int unit = TokenKind.UNIT_LENGTH_PX]) : super(value, t, span, unit) {
-    assert(_unit == TokenKind.UNIT_VIEWPORT_VW ||
-        _unit == TokenKind.UNIT_VIEWPORT_VH ||
-        _unit == TokenKind.UNIT_VIEWPORT_VMIN ||
-        _unit == TokenKind.UNIT_VIEWPORT_VMAX);
+    assert(unit == TokenKind.UNIT_VIEWPORT_VW ||
+        unit == TokenKind.UNIT_VIEWPORT_VH ||
+        unit == TokenKind.UNIT_VIEWPORT_VMIN ||
+        unit == TokenKind.UNIT_VIEWPORT_VMAX);
   }
 
   ViewportTerm clone() => new ViewportTerm(value, text, span, unit);
@@ -1094,7 +1097,7 @@
 
   GroupTerm(Span span) : _terms =  [], super(span);
 
-  add(LiteralTerm term) {
+  void add(LiteralTerm term) {
     _terms.add(term);
   }
 
@@ -1114,7 +1117,7 @@
 
   Expressions(Span span): super(span);
 
-  add(Expression expression) {
+  void add(Expression expression) {
     expressions.add(expression);
   }
 
@@ -1151,13 +1154,13 @@
 }
 
 abstract class DartStyleExpression extends TreeNode {
-  static final int unknownType = 0;
-  static final int fontStyle = 1;
-  static final int marginStyle = 2;
-  static final int borderStyle = 3;
-  static final int paddingStyle = 4;
-  static final int heightStyle = 5;
-  static final int widthStyle = 6;
+  static const int unknownType = 0;
+  static const int fontStyle = 1;
+  static const int marginStyle = 2;
+  static const int borderStyle = 3;
+  static const int paddingStyle = 4;
+  static const int heightStyle = 5;
+  static const int widthStyle = 6;
 
   final int _styleType;
   int priority;
@@ -1186,20 +1189,19 @@
 }
 
 class FontExpression extends DartStyleExpression {
-  Font font;
+  final Font font;
 
   //   font-style font-variant font-weight font-size/line-height font-family
-  FontExpression(Span span, {dynamic size, List<String>family,
-      int weight, String style, String variant, LineHeight lineHeight})
-      : super(DartStyleExpression.fontStyle, span) {
-    // TODO(terry): Only px/pt for now need to handle all possible units to
-    //              support calc expressions on units.
-    font = new Font(size : size is LengthTerm ? size.value : size,
-        family: family, weight: weight, style: style, variant: variant,
-        lineHeight: lineHeight);
-  }
+  // TODO(terry): Only px/pt for now need to handle all possible units to
+  //              support calc expressions on units.
+  FontExpression(Span span, {dynamic size, List<String> family,
+      int weight, String style, String variant, LineHeight lineHeight}) :
+        font = new Font(size : size is LengthTerm ? size.value : size,
+            family: family, weight: weight, style: style, variant: variant,
+            lineHeight: lineHeight),
+        super(DartStyleExpression.fontStyle, span);
 
-  merged(FontExpression newFontExpr) {
+  FontExpression merged(FontExpression newFontExpr) {
     if (this.isFont && newFontExpr.isFont) {
       return new FontExpression.merge(this, newFontExpr);
     }
@@ -1341,7 +1343,7 @@
   }
 
   HeightExpression clone() => new HeightExpression(span, height);
-  visit(VisitorBase visitor) => visitor.visitHeightExpression(this);
+  void visit(VisitorBase visitor) => visitor.visitHeightExpression(this);
 }
 
 class WidthExpression extends DartStyleExpression {
@@ -1359,7 +1361,7 @@
   }
 
   WidthExpression clone() => new WidthExpression(span, width);
-  visit(VisitorBase visitor) => visitor.visitWidthExpression(this);
+  void visit(VisitorBase visitor) => visitor.visitWidthExpression(this);
 }
 
 class PaddingExpression extends BoxExpression {
@@ -1393,5 +1395,5 @@
   PaddingExpression clone() =>
       new PaddingExpression(span, top: box.top, right: box.right,
       bottom: box.bottom, left: box.left);
-  visit(VisitorBase visitor) => visitor.visitPaddingExpression(this);
+  void visit(VisitorBase visitor) => visitor.visitPaddingExpression(this);
 }
diff --git a/lib/src/tree_base.dart b/lib/src/tree_base.dart
index e61a06c..6dc27b1 100644
--- a/lib/src/tree_base.dart
+++ b/lib/src/tree_base.dart
@@ -77,20 +77,6 @@
     writeln('${label}: ${v}');
   }
 
-  void writeList(String label, List<TreeNode> list) {
-    write('${label}: ');
-    if (list == null) {
-      buf.write('null');
-      buf.write('\n');
-    } else {
-      for (var item in list) {
-        buf.write(item.toString());
-        buf.write(', ');
-      }
-      buf.write('\n');
-    }
-  }
-
   void writeNodeList(String label, List<TreeNode> list) {
     writeln('${label} [');
     if (list != null) {
diff --git a/lib/src/tree_printer.dart b/lib/src/tree_printer.dart
index 3d381bd..3a8b748 100644
--- a/lib/src/tree_printer.dart
+++ b/lib/src/tree_printer.dart
@@ -129,9 +129,9 @@
 
   void visitStyletDirective(StyletDirective node) {
     heading('StyletDirective', node);
-    output.writeValue('dartClassName', node._dartClassName);
+    output.writeValue('dartClassName', node.dartClassName);
     output.depth++;
-    output.writeNodeList('rulesets', node._rulesets);
+    output.writeNodeList('rulesets', node.rulesets);
     output.depth--;
   }
 
@@ -200,7 +200,7 @@
   void visitDeclarationGroup(DeclarationGroup node) {
     heading('DeclarationGroup', node);
     output.depth++;
-    output.writeNodeList('declarations', node._declarations);
+    output.writeNodeList('declarations', node.declarations);
     output.depth--;
   }
 
@@ -208,7 +208,7 @@
     heading('MarginGroup', node);
     output.depth++;
     output.writeValue('@directive', node.margin_sym);
-    output.writeNodeList('declarations', node._declarations);
+    output.writeNodeList('declarations', node.declarations);
     output.depth--;
   }
 
diff --git a/lib/visitor.dart b/lib/visitor.dart
index 3b20a4e..6c009ff 100644
--- a/lib/visitor.dart
+++ b/lib/visitor.dart
@@ -111,7 +111,7 @@
 /** Base vistor class for the style sheet AST. */
 class Visitor implements VisitorBase {
   /** Helper function to walk a list of nodes. */
-  void _visitNodeList(list) {
+  void _visitNodeList(List<TreeNode> list) {
     // Don't use iterable otherwise the list can't grow while using Visitor.
     // It certainly can't have items deleted before the index being iterated
     // but items could be added after the index.
@@ -194,7 +194,7 @@
   }
 
   void visitStyletDirective(StyletDirective node) {
-    _visitNodeList(node._rulesets);
+    _visitNodeList(node.rulesets);
   }
 
   void visitNamespaceDirective(NamespaceDirective node) { }
@@ -230,7 +230,7 @@
   }
 
   void visitDeclarationGroup(DeclarationGroup node) {
-    _visitNodeList(node._declarations);
+    _visitNodeList(node.declarations);
   }
 
   void visitMarginGroup(MarginGroup node) => visitDeclarationGroup(node);
@@ -329,7 +329,7 @@
       visitSimpleSelector(node);
 
   void visitSelectorExpression(SelectorExpression node) {
-    _visitNodeList(node._expressions);
+    _visitNodeList(node.expressions);
   }
 
   void visitUnicodeRangeTerm(UnicodeRangeTerm node) { }
diff --git a/test/testing.dart b/test/testing.dart
index 168a4e5..002c452 100644
--- a/test/testing.dart
+++ b/test/testing.dart
@@ -18,7 +18,8 @@
  * CSS will allow any property/value pairs regardless of validity; all of our
  * tests (by default) will ensure that the CSS is really valid.
  */
-StyleSheet parseCss(String cssInput, {List errors, List opts}) =>
+StyleSheet parseCss(String cssInput, {List<Message> errors,
+  List<String> opts}) =>
   parse(cssInput, errors: errors, options: opts == null ?
       ['--no-colors', '--checked', '--warnings_as_errors', 'memory'] : opts);
 
@@ -27,13 +28,14 @@
  * CSS will allow any property/value pairs regardless of validity; all of our
  * tests (by default) will ensure that the CSS is really valid.
  */
-StyleSheet compileCss(String cssInput, {List errors, List opts,
+StyleSheet compileCss(String cssInput, {List<Message> errors, List<String> opts,
     bool polyfill: false, List<StyleSheet> includes: null}) =>
   compile(cssInput, errors: errors, options: opts == null ?
       ['--no-colors', '--checked', '--warnings_as_errors', 'memory'] : opts,
       polyfill: polyfill, includes: includes);
 
-StyleSheet polyFillCompileCss(input, {List errors, List opts}) =>
+StyleSheet polyFillCompileCss(input, {List<Message> errors,
+  List<String> opts}) =>
     compileCss(input, errors: errors, polyfill: true, opts: opts);
 
 /** CSS emitter walks the style sheet tree and emits readable CSS. */