Infer return types of closures (basic support).

pkg/front_end/testcases/inference/lambda_return_type.dart

Index: pkg/front_end/testcases/inference/lambda_return_type.dart
diff --git a/pkg/front_end/testcases/inference/lambda_return_type.dart b/pkg/front_end/testcases/inference/lambda_return_type.dart
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+// 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;
+
+typedef num FunctionReturningNum();
+
+main() {
+  int i = 1;
+  Object o = 1;
+  FunctionReturningNum a = /*@returnType=int*/ () => i;
+  FunctionReturningNum b = /*@returnType=Object*/ () => o;
+  FunctionReturningNum c = /*@returnType=int*/ () {
+    return i;
+  };
+  FunctionReturningNum d = /*@returnType=num*/ () {
+    return o;
+  };
+}