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

Revert commit 360d446.

Author: dnfield

lib/snapshot/BUILD.gn

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