Avoid crashing on function syntax.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 498b0740595b4ba699d05ea69b92bd7b71f3530f..3713341dd0c5d61f0c61cdc850950d932c5d849b 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -24,6 +24,7 @@ import '../builder/builder.dart'
         ClassBuilder,
         ConstructorReferenceBuilder,
         FormalParameterBuilder,
+        FunctionTypeBuilder,
         LibraryBuilder,
         MemberBuilder,
         MetadataBuilder,
@@ -209,6 +210,12 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
       List<FormalParameterBuilder> formals,
       int charOffset);
 
+  FunctionTypeBuilder addFunctionType(
+      T returnType,
+      List<TypeVariableBuilder> typeVariables,
+      List<FormalParameterBuilder> formals,
+      int charOffset);
+
   void addFactoryMethod(
       List<MetadataBuilder> metadata,
       int modifiers,