Fix some bad documentation links.

sdk/lib/core/symbol.dart

Index: sdk/lib/core/symbol.dart
diff --git a/sdk/lib/core/symbol.dart b/sdk/lib/core/symbol.dart
index fc391417b603223a27fa33ecdbed71de4b0d4dde..e399d9c0753a865f20a4860b4af3c1017fb92b6a 100644
--- a/sdk/lib/core/symbol.dart
+++ b/sdk/lib/core/symbol.dart
@@ -31,8 +31,8 @@ abstract class Symbol {
    * The following text is non-normative:
    *
    * Creating non-const Symbol instances may result in larger output.  If
-   * possible, use [MirrorsUsed] in "dart:mirrors" to specify which names might
-   * be passed to this constructor.
+   * possible, use `MirrorsUsed` from "dart:mirrors" to specify which names
+   * might be passed to this constructor.
    */
   const factory Symbol(String name) = internal.Symbol;