Introduce classes for string and name indexes

runtime/vm/bootstrap_nocore.cc

Index: runtime/vm/bootstrap_nocore.cc
diff --git a/runtime/vm/bootstrap_nocore.cc b/runtime/vm/bootstrap_nocore.cc
index 4176d0595d4a9efa8f73c8045e74c7cffa21da7a..c1e1e7441c0503c3d0758e683ec6331f4cf82e0d 100644
--- a/runtime/vm/bootstrap_nocore.cc
+++ b/runtime/vm/bootstrap_nocore.cc
@@ -95,7 +95,7 @@ RawError* BootstrapFromKernel(Thread* thread, kernel::Program* program) {
     dart_name = library.url();
     for (intptr_t j = 0; j < program->libraries().length(); ++j) {
       kernel::Library* kernel_library = program->libraries()[j];
-      intptr_t uri_index = kernel_library->import_uri();
+      kernel::StringIndex uri_index = kernel_library->import_uri();
       const String& kernel_name = reader.DartSymbol(uri_index);
       if (kernel_name.Equals(dart_name)) {
         reader.ReadLibrary(kernel_library);