Move mirrors from dartdoc to dart2js

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library mirrors_dart2js;
 
 import 'dart:io';
 import 'dart:uri';
 
 import '../../../../compiler/compiler.dart' as diagnostics;
@@ skipping 1515 matching lines @@
     if (node != null) {
       var span = mirrors.compiler.spanFromNode(node, script.uri);
       return new Dart2JsSourceLocation(script, span);
     } else {
       var span = mirrors.compiler.spanFromElement(_variable);
       return new Dart2JsSourceLocation(script, span);
     }
   }
 }