Update documentation on Symbol. Make validation match.

sdk/lib/internal/symbol.dart

Index: sdk/lib/internal/symbol.dart
diff --git a/sdk/lib/internal/symbol.dart b/sdk/lib/internal/symbol.dart
index 7a0369a9a5686a9b4fb926e632cd3eced29a11cf..5078cd3623f495f58570413f883d1b4c10156865 100644
--- a/sdk/lib/internal/symbol.dart
+++ b/sdk/lib/internal/symbol.dart
@@ -15,28 +15,24 @@ part of dart._internal;
 class Symbol implements core.Symbol {
   final String _name;
 
+  // Reserved words are not allowed as identifiers.
+  static const String reservedWord =
+      r'assert|break|c(?:a(?:se|tch)|lass|on(?:st|tinue))|d(?:efault|o)|'
+      r'e(?:lse|num|xtends)|f(?:alse|inal(?:ly)?|or)|i[fns]|n(?:ew|ull)|'
+      r'ret(?:hrow|urn)|s(?:uper|witch)|t(?:h(?:is|row)|r(?:ue|y))|'
+      r'v(?:ar|oid)|w(?:hile|ith)';
+
   static final RegExp validationPattern =
-      new RegExp(r'^(?:[a-zA-Z$][a-zA-Z$0-9_]*\.)*(?:[a-zA-Z$][a-zA-Z$0-9_]*=?|'
-                 r'-|'
-                 r'unary-|'
-                 r'\[\]=|'
-                 r'~|'
+      new RegExp(r'^(?:'
+                 r'[\-+*/%&|^]|'
+                 r'\[\]=?|'
                  r'==|'
-                 r'\[\]|'
-                 r'\*|'
-                 r'/|'
-                 r'%|'
-                 r'~/|'
-                 r'\+|'
-                 r'<<|'
-                 r'>>|'
-                 r'>=|'
-                 r'>|'
-                 r'<=|'
-                 r'<|'
-                 r'&|'
-                 r'\^|'
-                 r'\|'
+                 r'~/?|'
+                 r'<[<=]?|'
+                 r'>[>=]?|'
+                 r'unary-|'
+                 r'(?!(?:''$reservedWord'r')\b)[a-zA-Z$][\w$]*'
+                 r'(?:=?|(?:\.(?!(?:''$reservedWord'r')\b)[a-zA-Z$][\w$]*)*)'
                  r')$');
 
   external const Symbol(String name);