Avoid crashing on function syntax.

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

 // Copyright (c) 2016, 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.source_library_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../combinator.dart' show Combinator;
 
 import '../errors.dart' show internalError;
 
 import '../messages.dart' show warning;
 
 import '../import.dart' show Import;
 
 import 'source_loader.dart' show SourceLoader;
 
 import '../builder/scope.dart' show Scope;
 
 import '../builder/builder.dart'
     show
         Builder,
         ClassBuilder,
         ConstructorReferenceBuilder,
         FormalParameterBuilder,
+        FunctionTypeBuilder,
         LibraryBuilder,
         MemberBuilder,
         MetadataBuilder,
         PrefixBuilder,
         ProcedureBuilder,
         TypeBuilder,
         TypeDeclarationBuilder,
         TypeVariableBuilder,
         Unhandled;
 
@@ skipping 165 matching lines @@
       List<String> constants, int charOffset);
 
   void addFunctionTypeAlias(
       List<MetadataBuilder> metadata,
       T returnType,
       String name,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals,
       int charOffset);
 
+  FunctionTypeBuilder addFunctionType(
+      T returnType,
+      List<TypeVariableBuilder> typeVariables,
+      List<FormalParameterBuilder> formals,
+      int charOffset);
+
   void addFactoryMethod(
       List<MetadataBuilder> metadata,
       int modifiers,
       ConstructorReferenceBuilder name,
       List<FormalParameterBuilder> formals,
       AsyncMarker asyncModifier,
       ConstructorReferenceBuilder redirectionTarget,
       int charOffset,
       String nativeMethodName);
 
@@ skipping 279 matching lines @@
   }
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 }