Support parsing identifiers with escape codes in them

The output of `CssPrinter` will now also retain escape codes in identifiers.
This ensures they remain valid identifiers, as the escaped values may not parse
as valid identifiers.

The parser will also no longer accept an ID or class selector with space between
the first token (`#` or `.` respectively) and the identifier. The parser will
now fail immediately on these selector errors instead of attempting to recover.
Recovering in a robust manner is difficult given that this parser immediately
attempts to parse at the selector granularity, rather than first parsing the
style sheet into rules as described [here][parse-rule].

[parse-rule]: https://www.w3.org/TR/css-syntax-3/#consume-a-qualified-rule

Fixes https://github.com/dart-lang/csslib/issues/58.

Author: leonsenft

lib/parser.dart

@@ -1307,12 +1307,14 @@ class _Parser {
     var selectors = <Selector>[];
     var start = _peekToken.span;
 
+    tokenizer.inSelector = true;
     do {
       var selector = processSelector();
       if (selector != null) {
         selectors.add(selector);
       }
     } while (_maybeEat(TokenKind.COMMA));
+    tokenizer.inSelector = false;
 
     if (selectors.isNotEmpty) {
       return SelectorGroup(selectors, _makeSpan(start));
@@ -1501,35 +1503,20 @@ class _Parser {
       case TokenKind.HASH:
         _eat(TokenKind.HASH);
 
-        var hasWhiteSpace = false;
         if (_anyWhiteSpaceBeforePeekToken(TokenKind.HASH)) {
-          _warning('Not a valid ID selector expected #id', _makeSpan(start));
-          hasWhiteSpace = true;
-        }
-        if (_peekIdentifier()) {
-          var id = identifier();
-          if (hasWhiteSpace) {
-            // Generate bad selector id (normalized).
-            id.name = ' ${id.name}';
-          }
-          return IdSelector(id, _makeSpan(start));
+          _error('Not a valid ID selector expected #id', _makeSpan(start));
+          return null;
         }
-        return null;
+        return IdSelector(identifier(), _makeSpan(start));
       case TokenKind.DOT:
         _eat(TokenKind.DOT);
 
-        var hasWhiteSpace = false;
         if (_anyWhiteSpaceBeforePeekToken(TokenKind.DOT)) {
-          _warning('Not a valid class selector expected .className',
+          _error('Not a valid class selector expected .className',
               _makeSpan(start));
-          hasWhiteSpace = true;
-        }
-        var id = identifier();
-        if (hasWhiteSpace) {
-          // Generate bad selector class (normalized).
-          id.name = ' ${id.name}';
+          return null;
         }
-        return ClassSelector(id, _makeSpan(start));
+        return ClassSelector(identifier(), _makeSpan(start));
       case TokenKind.COLON:
         // :pseudo-class ::pseudo-element
         return processPseudoSelector(start);

lib/src/tree.dart

@@ -20,7 +20,12 @@ class Identifier extends TreeNode {
   dynamic visit(VisitorBase visitor) => visitor.visitIdentifier(this);
 
   @override
-  String toString() => name;
+  String toString() {
+    // Try to use the identifier's original lexeme to preserve any escape codes
+    // as authored. The name, which may include escaped values, may no longer be
+    // a valid identifier.
+    return span?.text ?? name;
+  }
 }
 
 class Wildcard extends TreeNode {
@@ -274,7 +279,7 @@ class AttributeSelector extends SimpleSelector {
   String valueToString() {
     if (value != null) {
       if (value is Identifier) {
-        return value.name;
+        return value.toString();
       } else {
         return '"$value"';
       }

test/error_test.dart

@@ -134,37 +134,27 @@ void testBadSelectors() {
 
   // Invalid id selector.
   var input = '# foo { color: #ff00ff; }';
-  var stylesheet = parseCss(input, errors: errors);
+  parseCss(input, errors: errors);
 
-  expect(errors.isEmpty, false);
+  expect(errors, isNotEmpty);
   expect(errors[0].toString(), r'''
 error on line 1, column 1: Not a valid ID selector expected #id
   ╷
 1 │ # foo { color: #ff00ff; }
   │ ^
   ╵''');
-  expect(stylesheet != null, true);
-  expect(prettyPrint(stylesheet), r'''
-# foo {
-  color: #f0f;
-}''');
 
   // Invalid class selector.
   input = '. foo { color: #ff00ff; }';
-  stylesheet = parseCss(input, errors: errors..clear());
+  parseCss(input, errors: errors..clear());
 
-  expect(errors.isEmpty, false);
+  expect(errors, isNotEmpty);
   expect(errors[0].toString(), r'''
 error on line 1, column 1: Not a valid class selector expected .className
   ╷
 1 │ . foo { color: #ff00ff; }
   │ ^
   ╵''');
-  expect(stylesheet != null, true);
-  expect(prettyPrint(stylesheet), r'''
-. foo {
-  color: #f0f;
-}''');
 }
 
 /// Test for bad hex values.

test/escape_codes_test.dart

@@ -0,0 +1,28 @@
+import 'package:csslib/parser.dart';
+import 'package:test/test.dart';
+
+import 'testing.dart';
+
+void main() {
+  final errors = <Message>[];
+
+  tearDown(() {
+    errors.clear();
+  });
+
+  group('handles escape codes', () {
+    group('in an identifier', () {
+      test('with trailing space', () {
+        final selectorAst = selector(r'.\35 00px', errors: errors);
+        expect(errors, isEmpty);
+        expect(compactOuptut(selectorAst), r'.\35 00px');
+      });
+
+      test('in an attribute selector value', () {
+        final selectorAst = selector(r'[elevation=\31]', errors: errors);
+        expect(errors, isEmpty);
+        expect(compactOuptut(selectorAst), r'[elevation=\31]');
+      });
+    });
+  });
+}