Allow null in Dart_SetRootLibrary.

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
diff --git a/runtime/vm/dart_api_impl_test.cc b/runtime/vm/dart_api_impl_test.cc
index d39e165391861bdb1ed2a4fab2d120bb64e96cac..1b5a5370e8d7c7f4df0a05eaba9942d2d474da20 100644
--- a/runtime/vm/dart_api_impl_test.cc
+++ b/runtime/vm/dart_api_impl_test.cc
@@ -5986,6 +5986,11 @@ TEST_CASE(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.
 }