Allow Dart_Null to be passed in for the import map in Dart_LoadLibrary and Dart_LoadScript.

vm/dart_api_impl.cc

Index: vm/dart_api_impl.cc
===================================================================
--- vm/dart_api_impl.cc	(revision 4552)
+++ vm/dart_api_impl.cc	(working copy)
@@ -2451,9 +2451,6 @@
     RETURN_TYPE_ERROR(source, String);
   }
   const Array& mapping_array = Api::UnwrapArrayHandle(import_map);
-  if (mapping_array.IsNull()) {
-    RETURN_TYPE_ERROR(import_map, Array);
-  }
   Library& library = Library::Handle(isolate->object_store()->root_library());
   if (!library.IsNull()) {
     const String& library_url = String::Handle(library.url());
@@ -2462,7 +2459,11 @@
   }
   isolate->set_library_tag_handler(handler);
   library = Library::New(url_str);
-  library.set_import_map(mapping_array);
+  if (mapping_array.IsNull()) {
+    library.set_import_map(Array::Handle(Array::Empty()));
+  } else {
+    library.set_import_map(mapping_array);
+  }
   library.Register();
   isolate->object_store()->set_root_library(library);
   Dart_Handle result;
@@ -2603,13 +2604,14 @@
     RETURN_TYPE_ERROR(source, String);
   }
   const Array& mapping_array = Api::UnwrapArrayHandle(import_map);
-  if (mapping_array.IsNull()) {
-    RETURN_TYPE_ERROR(import_map, Array);
-  }
   Library& library = Library::Handle(Library::LookupLibrary(url_str));
   if (library.IsNull()) {
     library = Library::New(url_str);
-    library.set_import_map(mapping_array);
+    if (mapping_array.IsNull()) {
+      library.set_import_map(Array::Handle(Array::Empty()));
+    } else {
+      library.set_import_map(mapping_array);
+    }
     library.Register();
   } else if (!library.LoadNotStarted()) {
     // The source for this library has either been loaded or is in the