Recover from bad supertypes.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_named_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_named_type_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_named_type_builder.dart
index f4f88acffe0ec954049e50166f87031a946b65d4..7dd7fb383ba3cc1c06f11ef3c04cf155eefb4a3c 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_named_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_named_type_builder.dart
@@ -14,6 +14,7 @@ import 'kernel_builder.dart'
         KernelClassBuilder,
         KernelInvalidTypeBuilder,
         KernelTypeBuilder,
+        LibraryBuilder,
         NamedTypeBuilder,
         TypeBuilder,
         TypeVariableBuilder;
@@ -37,27 +38,35 @@ class KernelNamedTypeBuilder
     return const DynamicType();
   }
 
-  Supertype handleMissingSuperType() {
+  Supertype handleMissingSupertype() {
     warning(fileUri, charOffset, "No type for: '$name'.");
     return null;
   }
 
-  DartType build() {
+  Supertype handleInvalidSupertype(LibraryBuilder library) {
+    String message = builder.isTypeVariable
+        ? "The type variable '$name' can't be used as supertype."
+        : "The type '$name' can't be used as supertype.";
+    library.addCompileTimeError(charOffset, message, fileUri);
+    return null;
+  }
+
+  DartType build(LibraryBuilder library) {
     if (name == "void") return const VoidType();
     if (name == "dynamic") return const DynamicType();
     if (builder == null) return handleMissingType();
-    return builder.buildType(arguments);
+    return builder.buildType(library, arguments);
   }
 
-  Supertype buildSupertype() {
+  Supertype buildSupertype(LibraryBuilder library) {
     if (name == "void") return null;
     if (name == "dynamic") return null;
-    if (builder == null) return handleMissingSuperType();
+    if (builder == null) return handleMissingSupertype();
     if (builder is KernelClassBuilder) {
       KernelClassBuilder builder = this.builder;
-      return builder.buildSupertype(arguments);
+      return builder.buildSupertype(library, arguments);
     } else {
-      return handleMissingSuperType();
+      return handleInvalidSupertype(library);
     }
   }