Perform type inference on explicitly typed fields.

pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
index 506c8858887eb69837e7f5cff5cbf242737f570a..00e8811b8d9ba3166a50770827e4d50c0c65285d 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
@@ -22,7 +22,7 @@ import 'package:front_end/src/fasta/source/source_library_builder.dart'
 import 'package:front_end/src/fasta/type_inference/type_inference_listener.dart'
     show TypeInferenceListener;
 
-import 'package:kernel/ast.dart' show Expression, Field, Name;
+import 'package:kernel/ast.dart' show DartType, Expression, Field, Name;
 
 import 'kernel_builder.dart'
     show Builder, FieldBuilder, KernelTypeBuilder, MetadataBuilder;
@@ -94,4 +94,7 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
       initializer = expression;
     }
   }
+
+  @override
+  DartType get builtType => field.type;
 }