Support explicit @charset.

lib/src/ast/sass/expression/string.dart

@@ -86,10 +86,12 @@ class StringExpression implements Expression {
   int _bestQuote() {
     var containsDoubleQuote = false;
     for (var value in text.contents) {
-      for (var i = 0; i < value.length; i++) {
-        var codeUnit = value.codeUnitAt(i);
-        if (codeUnit == $single_quote) return $double_quote;
-        if (codeUnit == $double_quote) containsDoubleQuote = true;
+      if (value is String) {
+        for (var i = 0; i < value.length; i++) {
+          var codeUnit = value.codeUnitAt(i);
+          if (codeUnit == $single_quote) return $double_quote;
+          if (codeUnit == $double_quote) containsDoubleQuote = true;
+        }
       }
     }
     return containsDoubleQuote ? $single_quote : $double_quote;

lib/src/parse/sass.dart

@@ -79,7 +79,8 @@ class SassParser extends StylesheetParser {
 
     var statements = <Statement>[];
     while (!scanner.isDone) {
-      statements.add(_child(statement));
+      var child = _child(statement);
+      if (child != null) statements.add(child);
       var indentation = _readIndentation();
       assert(indentation == 0);
     }

lib/src/parse/scss.dart

@@ -92,7 +92,8 @@ class ScssParser extends StylesheetParser {
               statements.add(_loudComment());
               break;
             default:
-              statements.add(statement());
+              var child = statement();
+              if (child != null) statements.add(child);
               break;
           }
           break;
@@ -103,7 +104,8 @@ class ScssParser extends StylesheetParser {
           break;
 
         default:
-          statements.add(statement());
+          var child = statement();
+          if (child != null) statements.add(child);
           break;
       }
     }

lib/src/parse/stylesheet.dart

@@ -71,7 +71,8 @@ abstract class StylesheetParser extends Parser {
 
   /// Consumes a statement that's allowed at the top level of the stylesheet.
   Statement _topLevelStatement() {
-    if (scanner.peekChar() == $at) return _atRule(_topLevelStatement);
+    if (scanner.peekChar() == $at)
+      return _atRule(_topLevelStatement, root: true);
     return _styleRule();
   }
 
@@ -301,13 +302,20 @@ abstract class StylesheetParser extends Parser {
   /// This consumes at-rules that are allowed at all levels of the document; the
   /// [child] parameter is called to consume any at-rules that are specifically
   /// allowed in the caller's context.
-  Statement _atRule(Statement child()) {
+  ///
+  /// If [root] is `true`, this parses at-rules that are allowed only at the
+  /// root of the stylesheet.
+  Statement _atRule(Statement child(), {bool root: false}) {
     var start = scanner.state;
     var name = _atRuleName();
 
     switch (name) {
       case "at-root":
         return _atRootRule(start);
+      case "charset":
+        if (!root) _disallowedAtRule(start);
+        string();
+        return null;
       case "content":
         return _contentRule(start);
       case "debug":
@@ -2193,5 +2201,8 @@ abstract class StylesheetParser extends Parser {
   List<Statement> children(Statement child());
 
   /// Consumes top-level statements.
+  ///
+  /// The [statement] callback may return `null`, indicating that a statement
+  /// was consumed that shouldn't be added to the AST.
   List<Statement> statements(Statement statement());
 }