Harmonize found string escaping with expectation escaping

Change how found strings are escaped when building syntax error
messages:

  * Do not escape non-ASCII characters (U+0100-U+FFFF). They are
    typically more readable in their raw form.

  * Escape DEL (U+007F). It is a control character.

  * Escape NUL (U+0000) as "\0", not "\x00".

  * Do not use less known shortcut escape sequences ("\b", "\f"), only the
    well-known ones ("\0", "\t", "\n", "\r").

These changes mirror expectation escaping changes done in
4fe682794d.

Part of work on #428.

lib/compiler/passes/generate-js.js

@@ -1075,31 +1075,18 @@ function generateJS(ast, options) {
       '    }',
       '',
       '    function buildMessage(expected, found) {',
-      '      function stringEscape(s) {',
+      '      function escape(s) {',
       '        function hex(ch) { return ch.charCodeAt(0).toString(16).toUpperCase(); }',
       '',
-      /*
-       * ECMA-262, 5th ed., 7.8.4: All characters may appear literally in a
-       * string literal except for the closing quote character, backslash,
-       * carriage return, line separator, paragraph separator, and line feed.
-       * Any character may appear in the form of an escape sequence.
-       *
-       * For portability, we also escape all control and non-ASCII characters.
-       * Note that "\0" and "\v" escape sequences are not used because JSHint
-       * does not like the first and IE the second.
-       */
       '        return s',
       '          .replace(/\\\\/g,   \'\\\\\\\\\')',   // backslash
       '          .replace(/"/g,    \'\\\\"\')',        // closing double quote
-      '          .replace(/\\x08/g, \'\\\\b\')',       // backspace
+      '          .replace(/\\0/g,   \'\\\\0\')',       // null
       '          .replace(/\\t/g,   \'\\\\t\')',       // horizontal tab
       '          .replace(/\\n/g,   \'\\\\n\')',       // line feed
-      '          .replace(/\\f/g,   \'\\\\f\')',       // form feed
       '          .replace(/\\r/g,   \'\\\\r\')',       // carriage return
-      '          .replace(/[\\x00-\\x07\\x0B\\x0E\\x0F]/g, function(ch) { return \'\\\\x0\' + hex(ch); })',
-      '          .replace(/[\\x10-\\x1F\\x80-\\xFF]/g,    function(ch) { return \'\\\\x\'  + hex(ch); })',
-      '          .replace(/[\\u0100-\\u0FFF]/g,         function(ch) { return \'\\\\u0\' + hex(ch); })',
-      '          .replace(/[\\u1000-\\uFFFF]/g,         function(ch) { return \'\\\\u\'  + hex(ch); });',
+      '          .replace(/[\\x01-\\x08\\x0B\\x0C\\x0E\\x0F]/g, function(ch) { return \'\\\\x0\' + hex(ch); })',
+      '          .replace(/[\\x10-\\x1F\\x7F-\\x9F]/g,        function(ch) { return \'\\\\x\'  + hex(ch); });',
       '      }',
       '',
       '      var expectedDescs = new Array(expected.length),',
@@ -1115,7 +1102,7 @@ function generateJS(ast, options) {
       '            + expectedDescs[expected.length - 1]',
       '        : expectedDescs[0];',
       '',
-      '      foundDesc = found ? "\\"" + stringEscape(found) + "\\"" : "end of input";',
+      '      foundDesc = found ? "\\"" + escape(found) + "\\"" : "end of input";',
       '',
       '      return "Expected " + expectedDesc + " but " + foundDesc + " found.";',
       '    }',

lib/parser.js

@@ -532,21 +532,18 @@
       }
 
       function buildMessage(expected, found) {
-        function stringEscape(s) {
+        function escape(s) {
           function hex(ch) { return ch.charCodeAt(0).toString(16).toUpperCase(); }
 
           return s
             .replace(/\\/g,   '\\\\')
             .replace(/"/g,    '\\"')
-            .replace(/\x08/g, '\\b')
+            .replace(/\0/g,   '\\0')
             .replace(/\t/g,   '\\t')
             .replace(/\n/g,   '\\n')
-            .replace(/\f/g,   '\\f')
             .replace(/\r/g,   '\\r')
-            .replace(/[\x00-\x07\x0B\x0E\x0F]/g, function(ch) { return '\\x0' + hex(ch); })
-            .replace(/[\x10-\x1F\x80-\xFF]/g,    function(ch) { return '\\x'  + hex(ch); })
-            .replace(/[\u0100-\u0FFF]/g,         function(ch) { return '\\u0' + hex(ch); })
-            .replace(/[\u1000-\uFFFF]/g,         function(ch) { return '\\u'  + hex(ch); });
+            .replace(/[\x01-\x08\x0B\x0C\x0E\x0F]/g, function(ch) { return '\\x0' + hex(ch); })
+            .replace(/[\x10-\x1F\x7F-\x9F]/g,        function(ch) { return '\\x'  + hex(ch); });
         }
 
         var expectedDescs = new Array(expected.length),
@@ -562,7 +559,7 @@
               + expectedDescs[expected.length - 1]
           : expectedDescs[0];
 
-        foundDesc = found ? "\"" + stringEscape(found) + "\"" : "end of input";
+        foundDesc = found ? "\"" + escape(found) + "\"" : "end of input";
 
         return "Expected " + expectedDesc + " but " + foundDesc + " found.";
       }