When translating from file to URI, make sure translation is reversible.

pkg/analyzer/test/source/package_map_resolver_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.
 
 library test.source.package_map_resolver;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 159 matching lines @@
       expect(uri, isNotNull);
       expect(uri.toString(), 'package:pkgB/src/libB.dart');
     }
     {
       Source source = _createFileSource('/no/such/file');
       Uri uri = resolver.restoreAbsolute(source);
       expect(uri, isNull);
     }
   }
 
+  void test_restoreAmbiguous() {
+    const file1 = '/foo1/lib/bar.dart';
+    const file2 = '/foo2/lib/bar.dart';
+    provider.newFile(file1, 'library bar');
+    provider.newFile(file2, 'library bar');
+    PackageMapUriResolver resolver =
+        new PackageMapUriResolver(provider, <String, List<Folder>>{
+      'foo': [
+          provider.getResource('/foo1/lib'),
+          provider.getResource('/foo2/lib')]
+    });
+    // Restoring file1 should yield a package URI, and that package URI should
+    // resolve back to file1.
+    Source source1 = _createFileSource(file1);
+    Uri uri1 = resolver.restoreAbsolute(source1);
+    expect(uri1.toString(), 'package:foo/bar.dart');
+    expect(resolver.resolveAbsolute(uri1).fullName, file1);
+    // Restoring file2 should not yield a package URI, because there is no URI
+    // that resolves to file2.
+    Source source2 = _createFileSource(file2);
+    expect(resolver.restoreAbsolute(source2), isNull);
+  }
+
   Source _createFileSource(String path) {
     return new NonExistingSource(path, UriKind.FILE_URI);
   }
 }