Use declared return type for inference of return and yield statements.

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 1c55c6ea42041591ca83e1db3a599335e3ad1a0b..d25cb22acdd472386424618257d653f827d0329e 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_listener.dart
@@ -13,7 +13,7 @@ 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, FunctionNode;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
@@ -433,6 +433,7 @@ class DietListener extends StackListener {
     assert(typeParameterScope != null);
     assert(formalParameterScope != null);
     parseFunctionBody(
+        builder.function,
         createListener(builder, typeParameterScope, builder.isInstanceMember,
             formalParameterScope),
         token,
@@ -515,7 +516,8 @@ class DietListener extends StackListener {
 
   AsyncMarker getAsyncMarker(StackListener listener) => listener.pop();
 
-  void parseFunctionBody(StackListener listener, Token token, MemberKind kind) {
+  void parseFunctionBody(FunctionNode function, StackListener listener,
+      Token token, MemberKind kind) {
     try {
       Parser parser = new Parser(listener);
       token = parser.parseFormalParametersOpt(token, kind);
@@ -534,7 +536,7 @@ class DietListener extends StackListener {
         listener.pop(); // separator before constructor initializers
       }
       listener.checkEmpty(token.charOffset);
-      listener.finishFunction(formals, asyncModifier, body);
+      listener.finishFunction(function, formals, asyncModifier, body);
     } on InputError {
       rethrow;
     } catch (e, s) {