First try to find the exact library for the given file URI.

pkg/analyzer/lib/src/generated/sdk_io.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 analyzer.src.generated.sdk_io;
 
 import 'dart:collection';
 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 452 matching lines @@
         String path = library.shortName;
         try {
           return new FileBasedSource(file, parseUriWithException(path));
         } on URISyntaxException catch (exception, stackTrace) {
           AnalysisEngine.instance.logger.logInformation(
               "Failed to create URI: $path",
               new CaughtException(exception, stackTrace));
           return null;
         }
       }
-      libraryPath = new JavaFile(libraryPath).getParent();
+    }
+    for (SdkLibrary library in _libraryMap.sdkLibraries) {
+      String libraryPath = new JavaFile(library.path).getParent();
       if (filePath.startsWith("$libraryPath${JavaFile.separator}")) {
         String path =
             "${library.shortName}/${filePath.substring(libraryPath.length + 1)}";
         try {
           return new FileBasedSource(file, parseUriWithException(path));
         } on URISyntaxException catch (exception, stackTrace) {
           AnalysisEngine.instance.logger.logInformation(
               "Failed to create URI: $path",
               new CaughtException(exception, stackTrace));
           return null;
@@ skipping 179 matching lines @@
     SdkLibrariesReader_LibraryBuilder libraryBuilder =
         new SdkLibrariesReader_LibraryBuilder(_useDart2jsPaths);
     // If any syntactic errors were found then don't try to visit the AST
     // structure.
     if (!errorListener.errorReported) {
       unit.accept(libraryBuilder);
     }
     return libraryBuilder.librariesMap;
   }
 }