Register toString dynamic invocation in the presence of string interpolation.

tests/compiler/dart2js/concrete_type_inference_test.dart

Index: tests/compiler/dart2js/concrete_type_inference_test.dart
diff --git a/tests/compiler/dart2js/concrete_type_inference_test.dart b/tests/compiler/dart2js/concrete_type_inference_test.dart
index 59c0b9cb2bf08ccff3053e8e404f2b82ad0b3153..99ad0f47c8663be8a6e44b943ea05eb11eb9d4a2 100644
--- a/tests/compiler/dart2js/concrete_type_inference_test.dart
+++ b/tests/compiler/dart2js/concrete_type_inference_test.dart
@@ -7,12 +7,6 @@
 #import('compiler_helper.dart');
 #import('parser_helper.dart');
 
-findElement(var compiler, String name) {
-  var element = compiler.mainApp.find(buildSourceString(name));
-  Expect.isNotNull(element, 'Could not locate $name.');
-  return element;
-}
-
 void compileAndFind(String code, String name,
                     check(compiler, element)) {
   Uri uri = new Uri.fromComponents(scheme: 'source');