Infer the return types of local functions where appropriate.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import '../fasta_codes.dart'
     show
         FastaMessage,
         codeConstFieldWithoutInitializer,
@@ skipping 2414 matching lines @@
       ..fileOffset = formals.charOffset
       ..fileEndOffset = endToken.charOffset));
   }
 
   @override
   void endFunctionDeclaration(Token token) {
     debugEvent("FunctionDeclaration");
     FunctionNode function = pop();
     exitLocalScope();
     var declaration = pop();
-    var returnType = pop() ?? const DynamicType();
+    var returnType = pop();
+    var hasImplicitReturnType = returnType == null;
+    returnType ??= const DynamicType();
     pop(); // Modifiers.
     exitFunction();
     if (declaration is FunctionDeclaration) {
+      KernelFunctionDeclaration.setHasImplicitReturnType(
+          declaration, hasImplicitReturnType);
       function.returnType = returnType;
       declaration.variable.type = function.functionType;
       declaration.function = function;
       function.parent = declaration;
     } else {
       // If [declaration] isn't a [FunctionDeclaration], it must be because
       // there was a compile-time error.
 
       // TODO(paulberry): ensure that when integrating with analyzer, type
       // inference is still performed for the dropped declaration.
@@ skipping 1233 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }