Move tar archive parsing to C++

sdk/lib/vmservice/asset.dart

Index: sdk/lib/vmservice/asset.dart
diff --git a/sdk/lib/vmservice/asset.dart b/sdk/lib/vmservice/asset.dart
index 7835e74442397d14d788f69124d2682246841b94..8155e8a8cea7558bdcfa882f04265bfbf81d0cdf 100644
--- a/sdk/lib/vmservice/asset.dart
+++ b/sdk/lib/vmservice/asset.dart
@@ -38,8 +38,8 @@ class Asset {
   }
 
   /// Call to request assets from the embedder.
-  static Map<String, Asset> request() {
-    Map<String, Asset> assets = new Map<String, Asset>();
+  static HashMap<String, Asset> request() {
+    HashMap<String, Asset> assets = new HashMap<String, Asset>();
     Uint8List tarBytes = _requestAssets();
     if (tarBytes == null) {
       return assets;