Use UnmodifiableMapView in compiler

sdk/lib/_internal/compiler/implementation/mirrors_used.dart

 // Copyright (c) 2013, 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 dart2js.mirrors_used;
 
 import 'dart2jslib.dart' show
     Compiler,
     CompilerTask,
     Constant,
@@ skipping 23 matching lines @@
     InterfaceType,
     TypeKind;
 
 import 'tree/tree.dart' show
     Import,
     LibraryTag,
     NamedArgument,
     NewExpression,
     Node;
 
-import 'resolution/resolution.dart' show
-    ConstantMapper;
-
 /**
  * Compiler task that analyzes MirrorsUsed annotations.
  *
  * When importing 'dart:mirrors', it is possible to annotate the import with
  * MirrorsUsed annotation.  This is a way to declare what elements will be
  * reflected on at runtime.  Such elements, even they would normally be
  * discarded by the implicit tree-shaking algorithm must be preserved in the
  * final output.
  *
  * Since some libraries cannot tell exactly what they will be reflecting on, it
@@ skipping 528 matching lines @@
       // @MirrorsUsed(targets: fisk)
       //                       ^^^^
       //
       // Instead of saying 'fisk' should pretty print the problematic constant
       // value.
       return spannable;
     }
     return node;
   }
 }