dart2js: add --preserve-uris flag.

tests/lib/mirrors/libraries_test.dart

Index: tests/lib/mirrors/libraries_test.dart
diff --git a/tests/lib/mirrors/libraries_test.dart b/tests/lib/mirrors/libraries_test.dart
index 1fd5d33bc1c405d9d5d71a0a1eab6341844331d3..f754c193ac0d12395fde0d254c668acb381ca4af 100644
--- a/tests/lib/mirrors/libraries_test.dart
+++ b/tests/lib/mirrors/libraries_test.dart
@@ -15,8 +15,17 @@ main() {
   Map<Uri, LibraryMirror> libraries = mirrors.libraries;
   Expect.isNotNull(libraries, 'libraries is null');
 
+  Expect.isTrue(libraries.isNotEmpty);
   LibraryMirror mirrorsLibrary = libraries[Uri.parse('dart:mirrors')];
-  Expect.isNotNull(mirrorsLibrary, 'mirrorsLibrary is null');
+  if (mirrorsLibrary == null) {
+    // In minified mode we don't preserve the URIs.
+    mirrorsLibrary = libraries.values
+        .firstWhere((LibraryMirror lm) => lm.simpleName == #dart.mirrors);
+    Uri uri = mirrorsLibrary.uri;
+    Expect.equals("https", uri.scheme);
+    Expect.equals("dartlang.org", uri.host);
+    Expect.equals("/dart2js-stripped-uri", uri.path);
+  }
 
   ClassMirror cls = mirrorsLibrary.declarations[#LibraryMirror];
   Expect.isNotNull(cls, 'cls is null');