Revert "Revert "Use single_root_scheme when compiling platform (#6402)" (#6431)" (#6473)

This reverts commit 5340f422, relands single root filesystem that is needed for reusable platform kernel files, that is safe to land now since dart roll c048bec5 has the fix for previously broken dependency file generation.

lib/snapshot/BUILD.gn

@@ -169,7 +169,9 @@ source_set("snapshot") {
 }
 
 compile_platform("non_strong_platform") {
-  libraries_specification_uri = "libraries.json"
+  single_root_scheme = "org-dartlang-sdk"
+  single_root_base = rebase_path("../../../")
+  libraries_specification_uri = "org-dartlang-sdk:///flutter/lib/snapshot/libraries.json"
 
   outputs = [
     "$root_out_dir/flutter_patched_sdk/platform.dill",
@@ -183,7 +185,9 @@ compile_platform("non_strong_platform") {
 }
 
 compile_platform("strong_platform") {
-  libraries_specification_uri = "libraries.json"
+  single_root_scheme = "org-dartlang-sdk"
+  single_root_base = rebase_path("../../../")
+  libraries_specification_uri = "org-dartlang-sdk:///flutter/lib/snapshot/libraries.json"
 
   outputs = [
     "$root_out_dir/flutter_patched_sdk/platform_strong.dill",