Respect setLibraryDebuggable

runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart

Index: runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
diff --git a/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart b/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
index fe51ff709b9772a9a7b85e67f8d740320ef23f21..0571376dc24bb1a97bb66af75c25a7f8b2a3efd7 100644
--- a/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
+++ b/runtime/observatory/tests/service/set_library_debuggable_rpc_test.dart
@@ -57,6 +57,29 @@ var tests = [
     }
     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);