Don't return values that aren't used.

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 207 matching lines @@
     if (optional("set", token)) return ProcedureKind.Setter;
     return internalError("Unhandled: ${token.value}");
   }
 
   @override
   void beginTopLevelMethod(Token token, Token name) {
     library.beginNestedDeclaration(hasMembers: false);
   }
 
   @override
-  ProcedureBuilder endTopLevelMethod(
+  void endTopLevelMethod(
       Token beginToken, Token getOrSet, Token endToken) {
     debugEvent("endTopLevelMethod");
     MethodBody kind = pop();
     AsyncMarker asyncModifier = pop();
     List<FormalParameterBuilder> formals = pop();
     List<TypeVariableBuilder> typeVariables = pop();
     String name = pop();
     TypeBuilder returnType = pop();
     int modifiers = Modifier.validate(pop(),
         isAbstract: kind == MethodBody.Abstract);
     List<MetadataBuilder> metadata = pop();
     checkEmpty();
-    return library.addProcedure(metadata, modifiers, returnType, name,
+    library.addProcedure(metadata, modifiers, returnType, name,
         typeVariables, formals, asyncModifier, computeProcedureKind(getOrSet),
         beginToken.charOffset);
   }
 
   @override
   void handleNoFunctionBody(Token token) {
     debugEvent("NoFunctionBody");
     push(MethodBody.Abstract);
   }
 
@@ skipping 333 matching lines @@
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }