Move the Kernel string offsets into the VM's heap.

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 44cff902480f0956bebf9f6a0cdeee44af1bbaef..4176d0595d4a9efa8f73c8045e74c7cffa21da7a 100644
--- a/runtime/vm/bootstrap_nocore.cc
+++ b/runtime/vm/bootstrap_nocore.cc
@@ -95,8 +95,8 @@ 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];
-      kernel::String* uri = kernel_library->import_uri();
-      const String& kernel_name = reader.DartSymbol(uri);
+      intptr_t uri_index = kernel_library->import_uri();
+      const String& kernel_name = reader.DartSymbol(uri_index);
       if (kernel_name.Equals(dart_name)) {
         reader.ReadLibrary(kernel_library);
         library.SetLoaded();