[VM / Service] Update core library source loading to use resolved_url

Source URIs for the core libraries in kernel take the form of
`org-dartlang-sdk://'. Previously, we had to do some manual
modifications to the URI stored by script objects which pointed to core
library scripts as they were often replaced with `dart:*` of being an
actual URI. Now that we have resolved_uri() in Script, we can use this
method to get the `org-dartlang-sdk://` URI and avoid the brittle
hacking of URIs.

For example, the resolved URI for the 'dart:io' script is
`org-dartlang-sdk:///sdk/lib/io/io.dart`.

Change-Id: I8e9f590be8e4b50d1476fc06a5c3928e63bef3b6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97660
Reviewed-by: Siva Annamalai <asiva@google.com>

runtime/vm/object.cc

@@ -9094,10 +9094,7 @@ bool Script::IsPartOfDartColonLibrary() const {
 #if !defined(DART_PRECOMPILED_RUNTIME)
 void Script::LoadSourceFromKernel(const uint8_t* kernel_buffer,
                                   intptr_t kernel_buffer_len) const {
-  const char* dart_prefix = "dart:";
-  const size_t dart_prefix_len = strlen(dart_prefix);
-  String& uri = String::Handle(url());
-  uri ^= String::SubString(uri, dart_prefix_len);
+  String& uri = String::Handle(resolved_url());
   String& source = String::Handle(kernel::KernelLoader::FindSourceForScript(
       kernel_buffer, kernel_buffer_len, uri));
   set_source(source);