Implement MirrorUsed.targets for libraries

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

Index: dart/sdk/lib/_internal/compiler/implementation/constants.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/constants.dart b/dart/sdk/lib/_internal/compiler/implementation/constants.dart
index 5e3a42942ea87b667f53b0d0062b0e1fb72eb5b9..4ff011ccd77651823df56bc82d548abf3de51400 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/constants.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/constants.dart
@@ -485,4 +485,15 @@ class ConstructedConstant extends ObjectConstant {
   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;
+  }
 }