Allowing `e` of any type in `void f(...) => e` in dart2js.

pkg/compiler/lib/src/typechecker.dart

 // Copyright (c) 2012, 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 dart2js.typechecker;
 
 import 'common/names.dart' show Identifiers;
 import 'common/resolution.dart' show Resolution;
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
@@ skipping 1689 matching lines @@
       if (executableContext.isGenerativeConstructor) {
         // The resolver already emitted an error for this expression.
       } else {
         if (currentAsyncMarker == AsyncMarker.ASYNC) {
           ResolutionInterfaceType futureOfFlattenedType =
               commonElements.futureType(types.flatten(expressionType));
           expressionType = futureOfFlattenedType;
         }
         if (expectedReturnType.isVoid &&
             !types.isAssignable(expressionType, const ResolutionVoidType())) {
-          reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID);
+          // In `void f(...) => e`, `e` can have any type.
+          if (!node.isArrowBody) {
+            reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID);
+          }
         } else {
           checkAssignable(expression, expressionType, expectedReturnType);
         }
       }
     } else if (currentAsyncMarker != AsyncMarker.SYNC) {
       // `return;` is allowed.
     } else if (!types.isAssignable(
         expectedReturnType, const ResolutionVoidType())) {
       // Let f be the function immediately enclosing a return statement of the
       // form 'return;' It is a static warning if both of the following
@@ skipping 333 matching lines @@
 
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     reporter.internalError(node,
         'Unexpected node ${node.getObjectDescription()} in the type checker.');
   }
 }