Introduce initial plumbing for type promotion in fasta.

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

Index: pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
index 8d17a8ab4286f0638c3c58d02067ee5d1a51433c..4a731400740f86d03c70ac95e16c3dbaa9a07277 100644
--- a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
@@ -8,6 +8,11 @@ export 'frontend_accessors.dart' show wrapInvalid;
 
 import 'frontend_accessors.dart' show Accessor, buildIsNull, makeLet;
 
+import 'package:front_end/src/fasta/builder/ast_factory.dart' show AstFactory;
+
+import 'package:front_end/src/fasta/type_inference/type_promotion.dart'
+    show TypePromoter;
+
 import 'package:kernel/ast.dart';
 
 import '../errors.dart' show internalError;
@@ -35,6 +40,12 @@ import '../names.dart' show callName;
 abstract class BuilderHelper {

[ahe, 2017/04/24 16:47:16]

BTW: If you have a suggestion for a better name for this interface, please let
me know.

[Paul Berry, 2017/04/24 18:20:30]

Ok, thanks.  So far I don't have any ideas.

   Uri get uri;
 
+  TypePromoter get typePromoter;
+
+  int get functionNestingLevel;
+
+  AstFactory get astFactory;
+
   Constructor lookupConstructor(Name name, {bool isSuper});
 
   Expression toSuperMethodInvocation(MethodInvocation node);
@@ -809,6 +820,7 @@ class NullAwarePropertyAccessor extends kernel.NullAwarePropertyAccessor
 }
 
 class VariableAccessor extends kernel.VariableAccessor with FastaAccessor {
+  @override
   final BuilderHelper helper;
 
   VariableAccessor(this.helper, int offset, VariableDeclaration variable,