Turn a Dart String + String into a JavaScript String + String.

tests/compiler/dart2js/string_add_test.dart

Index: tests/compiler/dart2js/string_add_test.dart
===================================================================
--- tests/compiler/dart2js/string_add_test.dart	(revision 27277)
+++ tests/compiler/dart2js/string_add_test.dart	(working copy)
@@ -8,12 +8,7 @@
 
 main() {
   asyncTest(() => compileAll(
-      r'''main() { return "${2}${true}${'a'}${3.14}"; }''').then((code) {
-    Expect.isTrue(code.contains(r'2truea3.14'));
+      r'''main() { return "foo" + "bar"; }''').then((code) {
+    Expect.isTrue(!code.contains(r'$add'));
   }));
-
-  asyncTest(() => compileAll(
-      r'''main() { return "foo ${new Object()}"; }''').then((code) {
-    Expect.isFalse(code.contains(r'concat'));
-  }));
 }