Update documentation on Symbol. Make validation match.

tests/lib/mirrors/symbol_validation_test.dart

Index: tests/lib/mirrors/symbol_validation_test.dart
diff --git a/tests/lib/mirrors/symbol_validation_test.dart b/tests/lib/mirrors/symbol_validation_test.dart
index 0fe3c45c3d412b142226754f67f66ee0473c4d18..732df336f5838a04b3a23269f43296b6b26b572b 100644
--- a/tests/lib/mirrors/symbol_validation_test.dart
+++ b/tests/lib/mirrors/symbol_validation_test.dart
@@ -36,4 +36,45 @@ main() {
 
   ['6', '0foo', ',', 'S with M', '_invalid&private']
       .forEach(invalidSymbol);
+
+  // Reserved words are not valid identifiers and therefore not valid symbols.
+  var reservedWords = [
+    "assert",
+    "break",
+    "case",
+    "catch",
+    "class",
+    "const",
+    "continue",
+    "default",
+    "do",
+    "else",
+    "enum",
+    "extends",
+    "false",
+    "final",
+    "finally",
+    "for",
+    "if",
+    "in",
+    "is",
+    "new",
+    "null",
+    "rethrow",
+    "return",
+    "super",
+    "switch",
+    "this",
+    "throw",
+    "true",
+    "try",
+    "var",
+    "void",
+    "while",
+    "with"
+  ];
+  reservedWords.expand((w) => [w, "$w=", "x.$w" , "$w.x", "x.$w.x"])
+               .forEach(invalidSymbol);
 }
+
+