Use a non-null TypeInferrer even in non-strong mode.

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:typed_data' show Uint8List;
 
@@ skipping 111 matching lines @@
           listener.uri = part.fileUri;
           parser.parseUnit(tokens);
         }
       }
     }
   }
 
   KernelTarget get target => super.target;
 
   DietListener createDietListener(LibraryBuilder library) {
-    var typeInferrer = target.strongMode
-        ? new KernelTypeInferrer(coreTypes, hierarchy, instrumentation)
-        : null;
+    var typeInferrer = new KernelTypeInferrer(
+        coreTypes, hierarchy, instrumentation, target.strongMode);
     return new DietListener(library, hierarchy, coreTypes, typeInferrer);
   }
 
   void resolveParts() {
     List<Uri> parts = <Uri>[];
     builders.forEach((Uri uri, LibraryBuilder library) {
       if (library is SourceLibraryBuilder) {
         if (library.isPart) {
           library.validatePart();
           parts.add(uri);
@@ skipping 219 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();
 }