Avoid crashing on function syntax.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index c2d6b22e1cc2e7d9f53019ed07ff0da8393b139b..0b8a198d2c9a97211b4419edf124d643142d5e79 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -34,6 +34,7 @@ import 'kernel_builder.dart'
         KernelFieldBuilder,
         KernelFormalParameterBuilder,
         KernelFunctionTypeAliasBuilder,
+        KernelFunctionTypeBuilder,
         KernelInvalidTypeBuilder,
         KernelMixinApplicationBuilder,
         KernelNamedMixinApplicationBuilder,
@@ -259,6 +260,15 @@ class KernelLibraryBuilder
     addBuilder(name, typedef, charOffset);
   }
 
+  KernelFunctionTypeBuilder addFunctionType(
+      KernelTypeBuilder returnType,
+      List<TypeVariableBuilder> typeVariables,
+      List<FormalParameterBuilder> formals,
+      int charOffset) {
+    return new KernelFunctionTypeBuilder(
+        charOffset, fileUri, returnType, typeVariables, formals);
+  }
+
   KernelFormalParameterBuilder addFormalParameter(
       List<MetadataBuilder> metadata,
       int modifiers,