[build] Fix incorrect output path in ddc's package_kernel_outline template.

Change-Id: I6c2f03c21437c2068c3ca36dbcc7a5bfd7e5f494
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433860
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Derek Xu <derekx@google.com>

utils/ddc/BUILD.gn

@@ -267,7 +267,7 @@ template("package_kernel_outline") {
          "Need 'package' in $target_name (the name of the package)")
 
   module = invoker.package
-  output = "$target_gen_dir/${module}_outline"
+  output = "$target_gen_dir/${module}_outline.dill"
   sdk_outline = rebase_path(sdk_outline_dill)
 
   package_config =
@@ -291,7 +291,7 @@ template("package_kernel_outline") {
       "--source",
       "package:$module/$module.dart",
       "--output",
-      rebase_path("$output.dill"),
+      rebase_path(output),
     ]
     if (defined(invoker.extra_libraries)) {
       foreach(lib, invoker.extra_libraries) {