Allow `return;` in async method.

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.
 
 part of dart2js;
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
@@ skipping 1592 matching lines @@
         if (currentAsyncMarker == AsyncMarker.ASYNC) {
           expressionType = coreTypes.futureType(types.flatten(expressionType));
         }
         if (expectedReturnType.isVoid &&
             !types.isAssignable(expressionType, const VoidType())) {
           reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID);
         } else {
           checkAssignable(expression, expressionType, expectedReturnType);
         }
       }
-
+    } else if (currentAsyncMarker == AsyncMarker.ASYNC) {
+      // `return;` is allowed.
     } else if (!types.isAssignable(expectedReturnType, const VoidType())) {
       // Let f be the function immediately enclosing a return statement of the
       // form 'return;' It is a static warning if both of the following
       // conditions hold:
       // - f is not a generative constructor.
       // - The return type of f may not be assigned to void.
       reportTypeWarning(node, MessageKind.RETURN_NOTHING,
                         {'returnType': expectedReturnType});
     }
     return const StatementType();
@@ skipping 234 matching lines @@
 
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     compiler.internalError(node,
         'Unexpected node ${node.getObjectDescription()} in the type checker.');
   }
 }