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

pkg/analyzer/lib/source/package_map_resolver.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 source.package_map_resolver;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/asserts.dart' as asserts;
 
@@ skipping 61 matching lines @@
     // This helps provide more meaningful error messages to users
     // (a missing file error, as opposed to an invalid URI error).
     return new NonExistingSource(uri.toString(), UriKind.PACKAGE_URI);
   }
 
   @override
   Uri restoreAbsolute(Source source) {
     String sourcePath = source.fullName;
     for (String pkgName in packageMap.keys) {
       List<Folder> pkgFolders = packageMap[pkgName];
-      for (Folder pkgFolder in pkgFolders) {
+      for (int i = 0; i < pkgFolders.length; i++) {
+        Folder pkgFolder = pkgFolders[i];
         String pkgFolderPath = pkgFolder.path;
-        if (sourcePath.startsWith(pkgFolderPath)) {
-          String relPath = sourcePath.substring(pkgFolderPath.length);
-          return Uri.parse('$PACKAGE_SCHEME:$pkgName$relPath');
+        // TODO(paulberry): figure out the right thing to do for Windows.

[Paul Berry, 2015/02/24 21:58:16]

Note: AFAICT this function is broken for Windows both before and after this CL,
since it doesn't translate between '/' and '\' file separators.  In the interest
of mitigating bug 22555 as quickly as possible, I'd like to address Windows in a
separate CL.

+        if (sourcePath.startsWith(pkgFolderPath + '/')) {
+          String relPath = sourcePath.substring(pkgFolderPath.length + 1);
+          if (_isReversibleTranslation(pkgFolders, i, relPath)) {
+            return Uri.parse('$PACKAGE_SCHEME:$pkgName/$relPath');
+          }
         }
       }
     }
     return null;
   }
 
   /**
+   * A translation from file path to package URI has just been found for
+   * using the [packageDirIndex]th element of [packageDirs], and appending the
+   * relative path [relPath].  Determine whether the translation is reversible;
+   * that is, whether translating the package URI pack to a file path will
+   * produce the file path we started with.
+   */
+  bool _isReversibleTranslation(List<Folder> packageDirs, int packageDirIndex,
+      String relPath) {
+    // The translation is reversible provided there is no prior element of
+    // [packageDirs] containing a file matching [relPath].
+    for (int i = 0; i < packageDirIndex; i++) {
+      Folder packageDir = packageDirs[i];
+      if (packageDir.exists) {
+        Resource result = packageDir.getChild(relPath);
+        if (result is File && result.exists) {
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
+  /**
    * Returns `true` if [uri] is a `package` URI.
    */
   static bool isPackageUri(Uri uri) {
     return uri.scheme == PACKAGE_SCHEME;
   }
 }