Make extra sure that _getMainClosure is set to the correct value.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index f70e50d2d2e01af2c1ca7aba529332d921f2ce95..2ee67f822627ca8fd84fb2c5c86601004e9c66ad 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -229,6 +229,8 @@ class KernelTarget extends TargetImplementation {
       finishAllConstructors();
       loader.finishNativeMethods();
       transformMixinApplications();
+      // TODO(ahe): Don't call this from two different places.
+      setup_builtin_library.transformProgram(program);
       errors.addAll(loader.collectCompileTimeErrors().map((e) => e.format()));
       if (errors.isNotEmpty) {
         return handleInputError(uri, null, isFullProgram: true);