Allow null in Dart_SetRootLibrary.

runtime/vm/dart_api_impl_test.cc

 // 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.
 
 #include "bin/builtin.h"
 #include "vm/compiler.h"
 #include "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 #include "include/dart_tools_api.h"
@@ skipping 5968 matching lines @@
   lib_uri = Dart_LibraryUrl(root_lib);
   EXPECT_VALID(Dart_StringToCString(lib_uri, &uri_cstr));
   EXPECT_STREQ(TestCase::url(), uri_cstr);  // Root library didn't change.
 
   result = Dart_SetRootLibrary(core_lib);
   EXPECT_VALID(result);
   root_lib = Dart_RootLibrary();
   lib_uri = Dart_LibraryUrl(root_lib);
   EXPECT_VALID(Dart_StringToCString(lib_uri, &uri_cstr));
   EXPECT_STREQ("dart:core", uri_cstr);  // Root library did change.
+
+  result = Dart_SetRootLibrary(Dart_Null());
+  EXPECT_VALID(result);
+  root_lib = Dart_RootLibrary();
+  EXPECT(Dart_IsNull(root_lib));  // Root library did change.
 }
 
 
 static int index = 0;
 
 
 static Dart_Handle import_library_handler(Dart_LibraryTag tag,
                                           Dart_Handle library,
                                           Dart_Handle url) {
   if (tag == Dart_kCanonicalizeUrl) {
@@ skipping 4155 matching lines @@
   result = Dart_Invoke(lib,
                        NewString("foozoo"),
                        0,
                        NULL);
   EXPECT(Dart_IsError(result));
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart