Introduce initial plumbing for type promotion in fasta.

pkg/front_end/lib/src/fasta/source/diet_listener.dart

Index: pkg/front_end/lib/src/fasta/source/diet_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/diet_listener.dart b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
index 750a3d838cf13b243400c8b381329a63b0dad209..36b7c6ce224ee725fb4dad5facfd5a1bd001090b 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -393,6 +393,7 @@ class DietListener extends StackListener {
   StackListener createListener(
       MemberBuilder builder, Scope memberScope, bool isInstanceMember,
       [Scope formalParameterScope]) {
+    var typeInferrer = typeInferenceEngine.createLocalTypeInferrer(uri);
     return new BodyBuilder(
         library,
         builder,
@@ -403,7 +404,7 @@ class DietListener extends StackListener {
         currentClass,
         isInstanceMember,
         uri,
-        typeInferenceEngine.createLocalTypeInferrer(uri),
+        typeInferrer,
         new KernelAstFactory())
       ..constantExpressionRequired = builder.isConstructor && builder.isConst;
   }