[kernel] Resolve single_base_root at GN time.

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Change-Id: Ia2bf1f76d736125f1bb1a01068ed671b47da7abd
Reviewed-on: https://dart-review.googlesource.com/63451
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
diff --git a/runtime/vm/BUILD.gn b/runtime/vm/BUILD.gn
index bfade78..4bbd984 100644
--- a/runtime/vm/BUILD.gn
+++ b/runtime/vm/BUILD.gn
@@ -305,7 +305,7 @@
 
 compile_platform("vm_legacy_platform") {
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [
@@ -320,7 +320,7 @@
   add_implicit_vm_platform_dependency = false
 
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [
diff --git a/utils/compiler/BUILD.gn b/utils/compiler/BUILD.gn
index 53197d5..8eab081 100644
--- a/utils/compiler/BUILD.gn
+++ b/utils/compiler/BUILD.gn
@@ -82,7 +82,7 @@
 
 compile_platform("compile_dart2js_platform") {
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [
@@ -98,7 +98,7 @@
 
 compile_platform("compile_dart2js_platform_strong") {
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [
@@ -115,7 +115,7 @@
 
 compile_platform("compile_dart2js_server_platform") {
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [
@@ -131,7 +131,7 @@
 
 compile_platform("compile_dart2js_server_platform_strong") {
   single_root_scheme = "org-dartlang-sdk"
-  single_root_base = "../../"
+  single_root_base = rebase_path("../../")
   libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
 
   outputs = [