Implement MirrorUsed.targets for libraries

dart/sdk/lib/_internal/compiler/implementation/constants.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.
 
 part of dart2js;
 
 abstract class ConstantVisitor<R> {
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
   R visitInt(IntConstant constant);
@@ skipping 467 matching lines @@
     if (fields.length != other.fields.length) return false;
     for (int i = 0; i < fields.length; i++) {
       if (fields[i] != other.fields[i]) return false;
     }
     return true;
   }
 
   List<Constant> getDependencies() => fields;
 
   accept(ConstantVisitor visitor) => visitor.visitConstructed(this);
+
+  Map<Element, Constant> get fieldElements {
+    // TODO(ahe): Refactor constant system to store this information directly.
+    ClassElement classElement = type.element;
+    int count = 0;
+    Map<Element, Constant> result = new Map<Element, Constant>();
+    classElement.implementation.forEachInstanceField((holder, field) {
+      result[field] = fields[count++];
+    }, includeSuperAndInjectedMembers: true);
+    return result;
+  }
 }