Add type inference for `this` expressions.

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

 // Copyright (c) 2017, 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.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 1502 matching lines @@
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     // TODO(scheglov): implement.
     return typeNeeded ? const DynamicType() : null;
   }
 }
 
 /// Shadow object for [ThisExpression].
 class KernelThisExpression extends ThisExpression implements KernelExpression {
   @override
   void _collectDependencies(KernelDependencyCollector collector) {
-    // TODO(paulberry): figure out the right thing to do here.
-    throw 'TODO(paulberry)';
+    // Field initializers are not allowed to refer to [this].  But if it
+    // happens, we can still proceed; no additional type inference dependencies
+    // are introduced.
   }
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    // TODO(scheglov): implement.
-    return typeNeeded ? const DynamicType() : null;
+    return typeNeeded ? (inferrer.thisType ?? const DynamicType()) : null;
   }
 }
 
 /// Shadow object for [Throw].
 class KernelThrow extends Throw implements KernelExpression {
   KernelThrow(Expression expression) : super(expression);
 
   @override
   void _collectDependencies(KernelDependencyCollector collector) {
     // No inference dependencies.
@@ skipping 20 matching lines @@
 
   @override
   FieldNode createFieldNode(KernelField field) {
     FieldNode fieldNode = new FieldNode(this, field);
     field._fieldNode = fieldNode;
     return fieldNode;
   }
 
   @override
   KernelTypeInferrer createLocalTypeInferrer(
-      Uri uri, TypeInferenceListener listener) {
-    return new KernelTypeInferrer._(this, uri.toString(), listener, false);
+      Uri uri, TypeInferenceListener listener, InterfaceType thisType) {
+    return new KernelTypeInferrer._(
+        this, uri.toString(), listener, false, thisType);
   }
 
   @override
-  KernelTypeInferrer createTopLevelTypeInferrer(
-      KernelField field, TypeInferenceListener listener) {
+  KernelTypeInferrer createTopLevelTypeInferrer(KernelField field,
+      TypeInferenceListener listener, InterfaceType thisType) {
     return field._typeInferrer =
-        new KernelTypeInferrer._(this, field.fileUri, listener, true);
+        new KernelTypeInferrer._(this, field.fileUri, listener, true, thisType);
   }
 
   @override
   bool fieldHasInitializer(KernelField field) {
     return field.initializer != null;
   }
 
   @override
   DartType getFieldDeclaredType(KernelField field) {
     return field._implicitlyTyped ? null : field.type;
@@ skipping 20 matching lines @@
   }
 }
 
 /// Concrete implementation of [TypeInferrer] specialized to work with kernel
 /// objects.
 class KernelTypeInferrer extends TypeInferrerImpl {
   @override
   final typePromoter = new KernelTypePromoter();
 
   KernelTypeInferrer._(KernelTypeInferenceEngine engine, String uri,
-      TypeInferenceListener listener, bool topLevel)
-      : super(engine, uri, listener, topLevel);
+      TypeInferenceListener listener, bool topLevel, InterfaceType thisType)
+      : super(engine, uri, listener, topLevel, thisType);
 
   @override
   Expression getFieldInitializer(KernelField field) {
     return field.initializer;
   }
 
   @override
   DartType inferExpression(
       Expression expression, DartType typeContext, bool typeNeeded) {
     if (expression is KernelExpression) {
@@ skipping 305 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }