Infer return types of closures (basic support).

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

Index: pkg/front_end/testcases/inference/lambda_return_type.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/lambda_return_type.dart.strong.expect b/pkg/front_end/testcases/inference/lambda_return_type.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..61410a53b210e1460eba3fa4a138ebedd3c7fb08
--- /dev/null
+++ b/pkg/front_end/testcases/inference/lambda_return_type.dart.strong.expect
@@ -0,0 +1,16 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  core::int i = 1;
+  core::Object o = 1;
+  () → core::num a = () → core::int => i;
+  () → core::num b = () → core::Object => o;
+  () → core::num c = () → core::int {
+    return i;
+  };
+  () → core::num d = () → core::num {
+    return o;
+  };
+}