Deal with bad supertypes.

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 d04bc35fb942f071386d3df0dbcd118947d3e234..0dff1fd1b3660ec77d664080befc0bc6d1574d65 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -76,6 +76,7 @@ import 'kernel_builder.dart'
     show
         Builder,
         ClassBuilder,
+        InvalidTypeBuilder,
         KernelClassBuilder,
         KernelLibraryBuilder,
         KernelNamedTypeBuilder,
@@ -444,6 +445,8 @@ class KernelTarget extends TargetImplementation {
                 builder.cls.mixin, constructor, substitutionMap));
           }
         }
+      } else if (supertype is InvalidTypeBuilder) {
+        builder.addSyntheticConstructor(makeDefaultConstructor());
       } else {
         internalError("Unhandled: ${supertype.runtimeType}");
       }