Add type inference for `this` expressions.

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/kernel/body_builder.dart' show BodyBuilder;
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show KernelField;
@@ skipping 55 matching lines @@
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder 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 typeInferenceEngine = library.loader.typeInferenceEngine;
       var listener = new TypeInferenceListener();
-      var typeInferrer =
-          typeInferenceEngine.createTopLevelTypeInferrer(field, listener);
+      var typeInferrer = typeInferenceEngine.createTopLevelTypeInferrer(
+          field, listener, field.enclosingClass?.thisType);

[scheglov, 2017/06/05 17:40:02]

Nit: The order is not logical - it would be better to have listener, then the
enclosing type, then the field.

[Paul Berry, 2017/06/05 18:37:16]

Thanks.  Fixed.

       var bodyBuilder = new BodyBuilder(
           library,
           this,
           memberScope,
           null,
           typeInferenceEngine.classHierarchy,
           typeInferenceEngine.coreTypes,
           currentClass,
           isInstanceMember,
           library.uri,
           typeInferrer);
       Parser parser = new Parser(bodyBuilder);
       Token token = parser.parseExpression(initializerToken);
       Expression expression = bodyBuilder.popForValue();
       bodyBuilder.checkEmpty(token.charOffset);
       initializer = expression;
     }
   }
 
   @override
   DartType get builtType => field.type;
 }