Respect setLibraryDebuggable

runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 library set_library_debuggable_rpc_test;
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 39 matching lines @@
       expect(false, isTrue, reason:'Unreachable');
     } on ServerRpcException catch(e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kInvalidParams));
       expect(e.message,
              "setLibraryDebuggable: "
              "invalid 'libraryId' parameter: libraries/9999999");
     }
     expect(caughtException, isTrue);
   },
+
+  // illegal (dart:_*) library.
+  (Isolate isolate) async {
+    await isolate.load();
+    Library dartInternal = isolate.libraries.firstWhere(
+      (Library library) => library.uri == 'dart:_internal');
+    var params = {
+      'libraryId': dartInternal.id,
+      'isDebuggable': false,
+    };
+    bool caughtException;
+    try {
+      await isolate.invokeRpcNoUpgrade('setLibraryDebuggable', params);
+      expect(false, isTrue, reason:'Unreachable');
+    } on ServerRpcException catch(e) {
+      caughtException = true;
+      expect(e.code, equals(ServerRpcException.kInvalidParams));
+      expect(e.message,
+             "setLibraryDebuggable: "
+             "illegal 'libraryId' parameter: ${dartInternal.id}");
+    }
+    expect(caughtException, isTrue);
+  },
 ];
 
 main(args) async => runIsolateTests(args, tests);