[fuchsia] Account for the new Dart toolchain. (flutter#4271)

build/flutter_app.gni

@@ -5,6 +5,7 @@
 assert(is_fuchsia)
 
 import("//build/dart/dart_package.gni")
+import("//build/dart/toolchain.gni")
 import("$flutter_root/lib/ui/dart_ui.gni")
 import("//topaz/public/dart-pkg/fuchsia/sdk_ext.gni")
 import("//topaz/public/dart-pkg/zircon/sdk_ext.gni")
@@ -62,7 +63,8 @@ template("flutter_jit_app") {
   flutter_core_snapshot_isolate_instructions =
       "$flutter_core_snapshot_gen_dir/isolate_snapshot_instructions.bin"
 
-  dot_packages = "$target_gen_dir/$dart_package_name.packages"
+  dart_target_gen_dir = get_label_info(":bogus($dart_toolchain)", "target_gen_dir")
+  dot_packages = "$dart_target_gen_dir/$dart_package_name.packages"
   bundle_depfile = "$bundle_path.d"
   snapshot_path = "$target_gen_dir/${target_name}_snapshot.bin"
   snapshot_depfile = "${snapshot_path}.d"
@@ -207,7 +209,8 @@ template("flutter_aot_app") {
     bundle_path = "$target_gen_dir/${target_name}.flx"
   }
 
-  dot_packages = "$target_gen_dir/$dart_package_name.packages"
+  dart_target_gen_dir = get_label_info(":bogus($dart_toolchain)", "target_gen_dir")
+  dot_packages = "$dart_target_gen_dir/$dart_package_name.packages"
   bundle_depfile = "$bundle_path.d"
   assembly_path = "$target_gen_dir/$target_name.S"
   assembly_depfile = "${assembly_path}.d"
@@ -257,7 +260,7 @@ template("flutter_aot_app") {
       "dart:mozart.internal," + rebase_path(mozart_dart_sdk_ext_lib),
       "--url-mapping",
       "dart:vmservice_sky," + rebase_path(
-              "$root_gen_dir/dart-pkg/sky_engine/sdk_ext/vmservice_io.dart"),
+              "$dart_root_gen_dir/dart-pkg/sky_engine/sdk_ext/vmservice_io.dart"),
 
       "--entry-points-manifest",
       rebase_path("$flutter_root/runtime/dart_vm_entry_points.txt"),

lib/snapshot/BUILD.gn

@@ -19,6 +19,7 @@ import("//third_party/dart/runtime/lib/typed_data_sources.gni")
 import("//third_party/dart/utils/generate_patch_sdk.gni")
 
 if (is_fuchsia) {
+  import("//build/dart/toolchain.gni")
   import("//topaz/public/dart-pkg/fuchsia/sdk_ext.gni")
   import("//topaz/public/dart-pkg/zircon/sdk_ext.gni")
   import("//topaz/public/lib/ui/flutter/sdk_ext/sdk_ext.gni")
@@ -46,7 +47,11 @@ if (is_fuchsia) {
     outputs = [
       dot_packages_file
     ]
-    deps = dart_deps
+    deps = []
+    foreach(dep, dart_deps) {
+      deps += [ "$dep($dart_toolchain)" ]
+    }
+
     depfile = "$dot_packages_file.d"
 
     script = "//build/dart/gen_dot_packages.py"
@@ -58,7 +63,7 @@ if (is_fuchsia) {
       "--root-build-dir",
       rebase_path(root_build_dir),
       "--root-gen-dir",
-      rebase_path(root_gen_dir),
+      rebase_path(dart_root_gen_dir),
       "--package-name",
       "snapshot_root",
       "--depfile",