Use single_root_scheme when compiling platform (flutter#6402)

Author: jensjoha

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",