Emit tab characters as \9 (#553)

Closes #552

Author: nex3

CHANGELOG.md

@@ -13,6 +13,9 @@
 * Match Ruby Sass's behavior in some edge-cases involving numbers with many
   significant digits.
 
+* Emit escaped tab characters in identifiers as `\9` rather than a backslash
+  followed by a literal tab.
+
 ### Command-Line Interface
 
 * The source map generated for a stylesheet read from standard input now uses a

lib/src/parse/parser.dart

@@ -413,7 +413,7 @@ abstract class Parser {
 
     if (identifierStart ? isNameStart(value) : isName(value)) {
       return String.fromCharCode(value);
-    } else if ((value <= 0x1F && value != $tab) ||
+    } else if (value <= 0x1F ||
         value == 0x7F ||
         (identifierStart && isDigit(value))) {
       var buffer = StringBuffer()..writeCharCode($backslash);