Add type inference for `this` expressions.

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 ab460fb3c76dd2472d150b57363e1c4281c6b40a..80d49e6ea2b904f9d2a281f17e3daec2dd18a4b9 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -10,7 +10,7 @@ import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart'
 import 'package:front_end/src/fasta/type_inference/type_inference_listener.dart'
     show TypeInferenceListener;
 
-import 'package:kernel/ast.dart' show AsyncMarker;
+import 'package:kernel/ast.dart' show AsyncMarker, Class, InterfaceType;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
@@ -403,8 +403,14 @@ class DietListener extends StackListener {
       MemberBuilder builder, Scope memberScope, bool isInstanceMember,
       [Scope formalParameterScope]) {
     var listener = new TypeInferenceListener();
+    var parent = builder.parent;
+    InterfaceType thisType;
+    if (parent is ClassBuilder) {

[ahe, 2017/06/06 11:16:52]

How about:

if (builder.isClassMember) {
  Class cls = builder.parent.target;
  thisType = cls.thisType;
}

I'm assuming that it's intentional that you get this-type regardless of
builder.isInstanceMember, as that provides better error recovery. I'll take care
of reporting incorrect accesses to this anyways. Perhaps add a comment about
that?

[Paul Berry, 2017/06/06 16:50:33]

Done.

+      Class class_ = parent.target;
+      thisType = class_.thisType;
+    }
     var typeInferrer =
-        typeInferenceEngine.createLocalTypeInferrer(uri, listener);
+        typeInferenceEngine.createLocalTypeInferrer(uri, listener, thisType);
     return new BodyBuilder(library, builder, memberScope, formalParameterScope,
         hierarchy, coreTypes, currentClass, isInstanceMember, uri, typeInferrer)
       ..constantExpressionRequired = builder.isConstructor && builder.isConst;