Update dart:mirrors to use Symbol.

dart/sdk/lib/core/symbol.dart

Index: dart/sdk/lib/core/symbol.dart
diff --git a/dart/sdk/lib/core/symbol.dart b/dart/sdk/lib/core/symbol.dart
index e2b686c2e160a3361644ccf186a174168a3b4861..163307fa49b199be27dfae492d507301cf4510f1 100644
--- a/dart/sdk/lib/core/symbol.dart
+++ b/dart/sdk/lib/core/symbol.dart
@@ -6,18 +6,13 @@ part of dart.core;
 
 /// Opaque name used by mirrors, invocations and [Function.apply].
 class Symbol {
-
   /**
    * Constructs a new Symbol.
    *
    * An [ArgumentError] is thrown if [name] starts with an underscore,
    * or if [name] is not a [String].  An [ArgumentError] is thrown if
-   * [name] is not an empty string and is not a valid identifier
-   * optionally followed by [:'=':].
+   * [name] is not an empty string and is not a valid qualified
+   * identifier optionally followed by [:'=':].
    */
-  external const Symbol(String name);
-
-  external bool operator ==(other);
-
-  external int get hashCode;
+  const factory Symbol(String name) = _collection_dev.Symbol;
 }