kernel->ssa: support string concatenations/interpolations

tests/compiler/dart2js/kernel/expressions_test.dart

+// Copyright (c) 2016, 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.
+
+import 'package:test/test.dart';
+
+import 'helper.dart' show check;
+
+main() {
+  group('string literals', () {
+    test('juxtaposition', () {
+      return check('main() { return "abc" "def"; }');
+    });
+    test('literal interpolation', () {
+      return check('main() { return "abc\${1}def"; }');
+    });
+    test('complex interpolation', () {
+      String code = '''
+foo() => 1;
+main() => "abc\${foo()}"
+          "\${foo()}def";''';
+      return check(code);
+    });
+  });
+}