Fix the resolution of uri-based part-of directives in the SDK (issue 29598)

pkg/analyzer/lib/src/dart/sdk/sdk.dart

Index: pkg/analyzer/lib/src/dart/sdk/sdk.dart
diff --git a/pkg/analyzer/lib/src/dart/sdk/sdk.dart b/pkg/analyzer/lib/src/dart/sdk/sdk.dart
index a6c49083fdeb69ed4bd6bf51aa7daa3fbd7d398a..cfed575fb9872503f05c0ce444b759fa6a5c8946 100644
--- a/pkg/analyzer/lib/src/dart/sdk/sdk.dart
+++ b/pkg/analyzer/lib/src/dart/sdk/sdk.dart
@@ -651,7 +651,13 @@ class FolderBasedDartSdk extends AbstractDartSdk {
     try {
       File file = libraryDirectory.getChildAssumingFile(library.path);
       if (!relativePath.isEmpty) {
-        file = file.parent.getChildAssumingFile(relativePath);
+        File relativeFile = file.parent.getChildAssumingFile(relativePath);
+        if (relativeFile.path == file.path) {
+          // The relative file is the library, so return a Source for the
+          // library rather than the part format.
+          return file.createSource(Uri.parse(library.shortName));
+        }
+        file = relativeFile;
       }
       return file.createSource(Uri.parse(dartUri));
     } on FormatException {