Don't expose private libraries with dart.library.X.

pkg/compiler/lib/src/apiimpl.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 leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:package_config/packages.dart';
@@ skipping 579 matching lines @@
 
   fromEnvironment(String name) {
     assert(invariant(NO_LOCATION_SPANNABLE,
         sdkLibraries != null, message: "setupSdk() has not been run"));
 
     var result = environment[name];
     if (result != null || environment.containsKey(name)) return result;
     if (!name.startsWith(dartLibraryEnvironmentPrefix)) return null;
 
     String libraryName = name.substring(dartLibraryEnvironmentPrefix.length);
+
+    // Private libraries are not exposed to the users.
+    if (libraryName.startsWith("_")) return null;
+
     if (sdkLibraries.containsKey(libraryName)) {
       // Dart2js always "supports" importing 'dart:mirrors' but will abort
       // the compilation at a later point if the backend doesn't support
       // mirrors. In this case 'mirrors' should not be in the environment.
-      if (name == dartLibraryEnvironmentPrefix + 'mirrors') {
+      if (libraryName == 'mirrors') {
         return backend.supportsReflection ? "true" : null;
       }
       return "true";
     }
     return null;
   }
 
   Uri lookupLibraryUri(String libraryName) {
     assert(invariant(NO_LOCATION_SPANNABLE,
         sdkLibraries != null, message: "setupSdk() has not been run"));
     return sdkLibraries[libraryName];
   }
 
   Uri resolvePatchUri(String libraryName) {
     return backend.resolvePatchUri(libraryName, platformConfigUri);
   }
 }