Stop working around issue 20504 in pub.

sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index 1eab02d6ad9e3fa0a60794586d796b17ff3e1880..613d94e890a15fd84540ccba6ecfc1e849679cc2 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -257,9 +257,6 @@ class AssetEnvironment {
           if (result.success) {
             log.message("Precompiled ${_formatExecutable(id)}.");
           } else {
-            // TODO(nweiz): Stop manually deleting this when issue 20504 is
-            // fixed.
-            deleteEntry(snapshotPath);
             throw new ApplicationException(
                 log.yellow("Failed to precompile "
                     "${_formatExecutable(id)}:\n") +