Fix for top-level inference error in SourceLoader.

pkg/front_end/lib/src/fasta/source/source_loader.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_loader;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show FileSystemException;
 
@@ skipping 28 matching lines @@
 import 'diet_parser.dart' show DietParser;
 
 import 'outline_builder.dart' show OutlineBuilder;
 
 import 'source_class_builder.dart' show SourceClassBuilder;
 
 import 'source_library_builder.dart' show SourceLibraryBuilder;
 
 class SourceLoader<L> extends Loader<L> {
   final Map<Uri, List<int>> sourceBytes = <Uri, List<int>>{};
-  final excludeSource = CompilerContext.current.options.excludeSource;
+  final bool excludeSource = CompilerContext.current.options.excludeSource;
 
   // Used when building directly to kernel.
   ClassHierarchy hierarchy;
   CoreTypes coreTypes;
 
   SourceLoader(TargetImplementation target) : super(target);
 
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
@@ skipping 302 matching lines @@
   void checkOverrides(List<SourceClassBuilder> sourceClasses) {
     assert(hierarchy != null);
     for (SourceClassBuilder builder in sourceClasses) {
       builder.checkOverrides(hierarchy);
     }
     ticker.logMs("Checked overrides");
   }
 
   List<Uri> getDependencies() => sourceBytes.keys.toList();
 }