Introduce initial plumbing for type promotion in fasta.

pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.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.
 
 library fasta.kernel_field_builder;
 
 import 'package:front_end/src/fasta/builder/ast_factory.dart' show AstFactory;
 
 import 'package:front_end/src/fasta/kernel/body_builder.dart' show BodyBuilder;
 
@@ skipping 64 matching lines @@
   Field get target => field;
 
   @override
   void prepareInitializerInference(TypeInferenceEngine typeInferenceEngine,
       LibraryBuilder library, ClassBuilder currentClass) {
     if (initializerToken != null) {
       var memberScope =
           currentClass == null ? library.scope : currentClass.scope;
       // TODO(paulberry): Is it correct to pass library.uri into BodyBuilder, or
       // should it be the part URI?
+      var typeInferrer = typeInferenceEngine.createTopLevelTypeInferrer(field);
       var bodyBuilder = new BodyBuilder(
           library,
           this,
           memberScope,
           null,
           typeInferenceEngine.classHierarchy,
           typeInferenceEngine.coreTypes,
           currentClass,
           isInstanceMember,
           library.uri,
-          typeInferenceEngine.createTopLevelTypeInferrer(field),
+          typeInferrer,
           astFactory,
           fieldDependencies: typeInferenceEngine.getFieldDependencies(field));
       Parser parser = new Parser(bodyBuilder);
       Token token = parser.parseExpression(initializerToken);
       Expression expression = bodyBuilder.popForValue();
       bodyBuilder.checkEmpty(token.charOffset);
       initializer = expression;
     }
   }
 }