Avoid crashing on function syntax.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
new file mode 100644
index 0000000000000000000000000000000000000000..7f7d4fc7227175be7881273f3fe19efabfa314e8
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_function_type_builder.dart
@@ -0,0 +1,26 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library fasta.kernel_function_type_builder;
+
+import 'package:kernel/ast.dart' show DartType, DynamicType, Supertype;
+
+import 'kernel_builder.dart'
+    show FunctionTypeBuilder, KernelTypeBuilder, LibraryBuilder;
+
+class KernelFunctionTypeBuilder extends FunctionTypeBuilder
+    implements KernelTypeBuilder {
+  KernelFunctionTypeBuilder(int charOffset, Uri fileUri,
+      KernelTypeBuilder returnType, List typeVariables, List formals)
+      : super(charOffset, fileUri, returnType, typeVariables, formals);
+
+  // TODO(ahe): Return a proper function type.
+  DartType build(LibraryBuilder library) => const DynamicType();
+
+  Supertype buildSupertype(LibraryBuilder library) {
+    library.addCompileTimeError(
+        charOffset, "Can't use a function type as supertype.", fileUri);
+    return null;
+  }
+}