dart2js: add --preserve-uris flag.

tests/compiler/dart2js/hide_uri_test.dart

+// Copyright (c) 2014, 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.
+
+import 'dart:async';
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'memory_compiler.dart' show compilerFor, OutputCollector;
+
+const MEMORY_SOURCE_FILES = const <String, String> {
+  'main.dart': """
+library main;
+
+@MirrorsUsed(targets: const ['main', 'lib'])
+import 'dart:mirrors';
+import 'lib.dart';
+
+
+class Subclass extends Super {
+  int _private;
+
+  int magic() => _private++;
+}
+
+main() {
+  var objects = [new Super(), new Subclass()];
+  print(currentMirrorSystem().findLibrary(#main).uri);
+}
+""",
+  'lib.dart': """
+library lib;
+
+class Super {
+  int _private;
+
+  int magic() => _private++;
+}
+"""
+};
+
+runTest(bool hideUris) {
+  OutputCollector collector = new OutputCollector();
+  var options = ["--minify"];
+  if (hideUris) options.add("--hide-uris");
+  var compiler = compilerFor(MEMORY_SOURCE_FILES,
+                             outputProvider: collector,
+                             options: options);
+  return compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) {
+    String jsOutput = collector.getOutput('', 'js');
+    Expect.equals(!hideUris, jsOutput.contains("main.dart"));
+    Expect.equals(!hideUris, jsOutput.contains("lib.dart"));

[blois, 2014/09/22 21:26:09]

I'm curious what the output is from an import such as 'foo/bar.dart'- ideally it
would be 'foo/bar.dart'. This allows resolving URIs relative to the entry
script.

[floitsch, 2014/09/22 21:28:10]

I'm pretty sure it's absolute.
Fwiw the VM has similar issues, and sometimes has relative and sometimes
absolute URIs.

+  });
+}
+
+void main() {
+  asyncStart();
+  new Future.value()
+    .then((_) => runTest(false))
+    .then((_) => runTest(true))
+    .whenComplete(asyncEnd);
+}