Fix paths when run from same directory as manifest

Towards #37610

When the `script_uri` argument to the VM has no path separators the
value returned from `DirName` is identical to the input, and it is not
safe to use it as a prefix for concatenating relative paths.

- Compare the dirname against the script URI and use an empty prefix
  when they match, indicating we were unable to find a directory.
- Rename `absolute_filename` to `resolved_filename` since it is not
  absolute in all cases.

Change-Id: I62e5b06d240b55b3aa9d97d61c003adb812485e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/110137
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Nate Bosch <[email protected]>

runtime/bin/dfe.cc

@@ -440,6 +440,9 @@ static bool TryReadKernelListBuffer(const char* script_uri,
                                     uint8_t** kernel_ir,
                                     intptr_t* kernel_ir_size) {
   const char* kernel_list_dirname = DartUtils::DirName(script_uri);
+  if (strcmp(kernel_list_dirname, script_uri) == 0) {
+    kernel_list_dirname = "";
+  }
   KernelIRNode* kernel_ir_head = nullptr;
   KernelIRNode* kernel_ir_tail = nullptr;
   // Add all kernels to the linked list
@@ -452,11 +455,11 @@ static bool TryReadKernelListBuffer(const char* script_uri,
     intptr_t this_kernel_size;
     uint8_t* this_buffer;
 
-    StringPointer absolute_filename(
+    StringPointer resolved_filename(
         File::IsAbsolutePath(filename)
             ? strdup(filename)
             : Utils::SCreate("%s%s", kernel_list_dirname, filename));
-    if (!TryReadFile(absolute_filename.c_str(), &this_buffer,
+    if (!TryReadFile(resolved_filename.c_str(), &this_buffer,
                      &this_kernel_size)) {
       return false;
     }