Introduce initial plumbing for type promotion in fasta.

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

Index: pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart b/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
index 54c6d36405444e6f331fed343315863f29a651a3..c6c863462a7869b91985c42ba02bda042137ef7a 100644
--- a/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
@@ -141,17 +141,24 @@ abstract class Accessor {
   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)