Introduce classes for string and name indexes

runtime/vm/bootstrap.cc

Index: runtime/vm/bootstrap.cc
diff --git a/runtime/vm/bootstrap.cc b/runtime/vm/bootstrap.cc
index 5a10c6534619664e89c3bdc20e06cd5d48c33227..a06710889ebc9278e42f76ffaa93f46fcc905302 100644
--- a/runtime/vm/bootstrap.cc
+++ b/runtime/vm/bootstrap.cc
@@ -367,7 +367,7 @@ static 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);