Perform type inference on explicitly typed fields.

pkg/front_end/lib/src/fasta/builder/field_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/field_builder.dart b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
index f8cfbc0e6f9f94318c955393d5a195df7936406d..d5a37b6b5d146ee66588bf9550d11c6956cb63ef 100644
--- a/pkg/front_end/lib/src/fasta/builder/field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
@@ -6,6 +6,8 @@ library fasta.field_builder;
 
 import 'builder.dart' show LibraryBuilder, MemberBuilder;
 
+import 'package:kernel/ast.dart' show DartType;
+
 abstract class FieldBuilder<T> extends MemberBuilder {
   final String name;
 
@@ -15,6 +17,8 @@ abstract class FieldBuilder<T> extends MemberBuilder {
       this.name, this.modifiers, LibraryBuilder compilationUnit, int charOffset)
       : super(compilationUnit, charOffset);
 
+  DartType get builtType;
+
   void set initializer(T value);
 
   bool get isField => true;