Add proper API for creating private symbols wrt a library.

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
diff --git a/runtime/lib/mirrors_impl.dart b/runtime/lib/mirrors_impl.dart
index e20aa8874d603d414c29eaca193a54e8a4bcd9d7..226c85dd733dccc6dac67bbd02f73b360eec30c6 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -289,7 +289,7 @@ class _LocalInstanceMirrorImpl extends _LocalObjectMirrorImpl
       _LocalInstanceMirrorImpl mirror =
           reflect(invocation);
       _invokeOnClosure = reflectClass(invocation.runtimeType)
-          .getField(const Symbol('_invokeOnClosure')).reflectee;
+          .getField(MirrorSystem.getSymbol('_invokeOnClosure', mirror.type.owner)).reflectee;
     }
     return _invokeOnClosure(reflectee, invocation);
   }
@@ -644,7 +644,7 @@ class _LocalClassMirrorImpl extends _LocalObjectMirrorImpl
     }
   }
 
-  String toString() => "ClassMirror on '${_n(simpleName)}'";
+  String toString() => "ClassMirror on '${MirrorSystem.getName(simpleName)}'";
 
   InstanceMirror newInstance(Symbol constructorName,
                              List positionalArguments,
@@ -1127,7 +1127,7 @@ class _LocalMethodMirrorImpl extends _LocalDeclarationMirrorImpl
     }
   }
 
-  String toString() => "MethodMirror on '${_n(simpleName)}'";
+  String toString() => "MethodMirror on '${MirrorSystem.getName(simpleName)}'";
 
   static dynamic _MethodMirror_owner(reflectee)
       native "MethodMirror_owner";
@@ -1172,7 +1172,7 @@ class _LocalVariableMirrorImpl extends _LocalDeclarationMirrorImpl
     return _type;
   }
 
-  String toString() => "VariableMirror on '${_n(simpleName)}'";
+  String toString() => "VariableMirror on '${MirrorSystem.getName(simpleName)}'";
 
   static _VariableMirror_type(reflectee)
       native "VariableMirror_type";