Fix a type inference corner case: return expression where void expected.

pkg/front_end/testcases/inference/block_bodied_lambdas_void_context.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.
+
+/*@testedFeatures=inference*/
+library test;
+
+f() {
+  List<int> o;
+  o. /*@target=Iterable::forEach*/ forEach(
+      /*@returnType=void*/ (/*@type=int*/ i) {
+    return i /*@target=num::+*/ + 1;
+  });
+}
+
+main() {}