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

pkg/analyzer/lib/src/generated/sdk_io.dart

Index: pkg/analyzer/lib/src/generated/sdk_io.dart
diff --git a/pkg/analyzer/lib/src/generated/sdk_io.dart b/pkg/analyzer/lib/src/generated/sdk_io.dart
index 6a1e38d1b38ba1844db12361be84db6fa11801d2..2b11478cec57308291380e4e887ace9d48c9ff63 100644
--- a/pkg/analyzer/lib/src/generated/sdk_io.dart
+++ b/pkg/analyzer/lib/src/generated/sdk_io.dart
@@ -470,7 +470,9 @@ class DirectoryBasedDartSdk implements DartSdk {
           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)}";