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

pkg/front_end/testcases/inference/lambda_void_context.dart.strong.expect

Index: pkg/front_end/testcases/inference/lambda_void_context.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/lambda_void_context.dart.strong.expect b/pkg/front_end/testcases/inference/lambda_void_context.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..13abe3f0dffa0f97a92e950d382910753d4ebcba
--- /dev/null
+++ b/pkg/front_end/testcases/inference/lambda_void_context.dart.strong.expect
@@ -0,0 +1,9 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method f() → dynamic {
+  core::List<core::int> o;
+  o.{core::Iterable::forEach}((core::int i) → core::int => i.{core::num::+}(1));
+}
+static method main() → dynamic {}