Fix compilation errors on Windows.

runtime/vm/dart_api_impl.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 "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
@@ skipping 4031 matching lines @@
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);
   const Library& library_vm = Api::UnwrapLibraryHandle(isolate, library);
   if (library_vm.IsNull()) {
     RETURN_TYPE_ERROR(isolate, library, Library);
   }
   const Library& import_vm = Api::UnwrapLibraryHandle(isolate, import);
   if (import_vm.IsNull()) {
     RETURN_TYPE_ERROR(isolate, import, Library);
   }
-  const String& prefix_vm = Dart_IsNull(prefix)
+  const Object& prefix_object =
+      Object::Handle(isolate, Api::UnwrapHandle(prefix));
+  const String& prefix_vm = prefix_object.IsNull()
       ? String::Handle(isolate, Symbols::New(""))
-      : Api::UnwrapStringHandle(isolate, prefix);
+      : String::Cast(prefix_object);
   if (prefix_vm.IsNull()) {
     RETURN_TYPE_ERROR(isolate, prefix, String);
   }
   const String& prefix_symbol =
       String::Handle(isolate, Symbols::New(prefix_vm));
   if (prefix_vm.Length() == 0) {
     library_vm.AddImport(import_vm);
   } else {
     LibraryPrefix& library_prefix = LibraryPrefix::Handle();
     library_prefix = library_vm.LookupLocalLibraryPrefix(prefix_symbol);
@@ skipping 86 matching lines @@
 DART_EXPORT void Dart_InitPerfEventsSupport(Dart_FileWriterFunction function) {
   Dart::set_perf_events_writer(function);
 }
 
 
 DART_EXPORT void Dart_InitFlowGraphPrinting(Dart_FileWriterFunction function) {
   Dart::set_flow_graph_writer(function);
 }
 
 }  // namespace dart