Make it possible to share the embedder's dart sources for the vmservice

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 8155e8a8cea7558bdcfa882f04265bfbf81d0cdf..b8f6c207f94dd431d232f2c2f5c4d0fd617afb24 100644
--- a/sdk/lib/vmservice/asset.dart
+++ b/sdk/lib/vmservice/asset.dart
@@ -59,6 +59,17 @@ class Asset {
   String toString() => '$name ($mimeType)';
 }
 
+HashMap<String, Asset> _assets;
+HashMap<String, Asset> get assets {
+  if (_assets == null) {
+    try {
+      _assets = Asset.request();
+    } catch (e) {
+      print('Could not load Observatory assets: $e');
+    }
+  }
+  return _assets;
+}
 
 class _ByteStream {
   final Uint8List bytes;