dart2js: add --preserve-uris flag.

sdk/lib/_internal/compiler/js_lib/js_mirrors.dart

 // Copyright (c) 2013, 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.
 
 library dart._js_mirrors;
 
 import 'shared/runtime_data.dart' as encoding;
 import 'shared/embedded_names.dart' show
     ALL_CLASSES,
     LAZIES,
@@ skipping 328 matching lines @@
                   this._isRoot,
                   this._globalObject)
       : super(simpleName) {
     preserveLibraryNames();
   }
 
   String get _prettyName => 'LibraryMirror';
 
   Uri get uri {
     preserveUris();
-    return _uri;
+    if (_uri != "") return _uri;
+    // The Uri has been compiled out. Create a URI from the simple name.
+    return new Uri(scheme: "missing", path: n(simpleName));
   }
 
   Symbol get qualifiedName => simpleName;
 
   List<JsMethodMirror> get _methods => _functionMirrors;
 
   Map<Symbol, ClassMirror> get __classes {
     if (_cachedClasses != null) return _cachedClasses;
     var result = new Map();
     for (String className in _classes) {
@@ skipping 2595 matching lines @@
   // have a part (following a '.') that starts with '_'.
   const int UNDERSCORE = 0x5f;
   if (name.isEmpty) return true;
   int index = -1;
   do {
     if (name.codeUnitAt(index + 1) == UNDERSCORE) return false;
     index = name.indexOf('.', index + 1);
   } while (index >= 0 && index + 1 < name.length);
   return true;
 }