Type inference for StringConcatenation.

pkg/front_end/testcases/inference/string_literal.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.md file.
+
+/*@testedFeatures=inference*/
+library test;
+
+var /*@topType=int*/ x = 1;
+var /*@topType=String*/ a = 'aaa';
+var /*@topType=String*/ b = 'b ${x} bb';
+var /*@topType=String*/ c = 'c ${x} cc' 'ccc';
+
+main() {
+  var /*@type=int*/ x = 1;
+  var /*@type=String*/ a = 'aaa';
+  var /*@type=String*/ b = 'b ${/*@promotedType=none*/x} bb';
+  var /*@type=String*/ c = 'c ${/*@promotedType=none*/x} cc' 'ccc';
+}