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

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

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index b52e4609143d472e8002034c69bb18cc802a9a5e..7799c30fc7f76c3d40c686e74537aedfa396ffb3 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -129,9 +129,8 @@ class SourceLoader<L> extends Loader<L> {
   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);
   }