Compute import paths using ImportElement and ExportElement

tests/compiler/dart2js/serialization/test_data.dart

Index: tests/compiler/dart2js/serialization/test_data.dart
diff --git a/tests/compiler/dart2js/serialization/test_data.dart b/tests/compiler/dart2js/serialization/test_data.dart
index 49f3f4454e60b89f2fb0fd3dfe041c8eff31202d..96efc314d69b20f952938857e13e33e98aeb1229 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -308,6 +308,14 @@ class S {
 class C = S with M;
 ''',
       }),
+
+  const Test('Import mirrors, thus checking import paths', const {
+    'main.dart': '''
+import 'dart:mirrors';
+main() {}
+''',
+  },
+      expectedWarningCount: 1),
 ];
 
 class Test {