Introduce initial plumbing for type promotion in fasta.

pkg/front_end/lib/src/fasta/kernel/frontend_accessors.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.
 
 /// A library to help transform compounds and null-aware accessors into
 /// let expressions.
 
 import 'package:front_end/src/fasta/kernel/fasta_accessors.dart'
     show BuilderHelper;
 
@@ skipping 123 matching lines @@
   /// At runtime, an exception will be thrown.
   makeInvalidRead() => new InvalidExpression();
 
   /// Returns an [Expression] representing a compile-time error wrapping
   /// [value].
   ///
   /// At runtime, [value] will be evaluated before throwing an exception.
   makeInvalidWrite(Expression value) => wrapInvalid(value);
 }
 
-class VariableAccessor extends Accessor {
+abstract class VariableAccessor extends Accessor {
   VariableDeclaration variable;
   DartType promotedType;
 
+  BuilderHelper get helper;
+
   VariableAccessor(this.variable, this.promotedType, int offset)
       : super(offset);
 
-  Expression _makeRead() =>
-      new VariableGet(variable, promotedType)..fileOffset = offset;
+  Expression _makeRead() {
+    var fact = helper.typePromoter
+        .getFactForAccess(variable, helper.functionNestingLevel);
+    var scope = helper.typePromoter.currentScope;
+    return helper.astFactory.variableGet(variable, fact, scope, offset);
+  }
 
   Expression _makeWrite(Expression value, bool voidContext) {
+    helper.typePromoter.mutateVariable(variable, helper.functionNestingLevel);
     return variable.isFinal || variable.isConst
         ? makeInvalidWrite(value)
         : new VariableSet(variable, value)
       ..fileOffset = offset;
   }
 }
 
 class PropertyAccessor extends Accessor {
   VariableDeclaration _receiverVariable;
   Expression receiver;
@@ skipping 346 matching lines @@
 
 VariableDeclaration makeOrReuseVariable(Expression value) {
   // TODO: Devise a way to remember if a variable declaration was reused
   // or is fresh (hence needs a let binding).
   return new VariableDeclaration.forValue(value);
 }
 
 Expression wrapInvalid(Expression e) {
   return new Let(new VariableDeclaration.forValue(e), new InvalidExpression());
 }