Use relative URIs for core library part-of statements.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 3ed4f7f9ff91d6620f17aa6dc7b6f30b6a84522c..a21b00c8c8d86f4abd710dbd7bb3bd19c64492a6 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -423,7 +423,7 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
         String myName = name == null ? "'$uri'" : "'${name}' ($uri)";
         addWarning(
             -1,
-            "Using '${part.fileUri}' as part of '$myName' but it's 'part of'"
+            "Using '${part.fileUri}' as part of '$myName' but its 'part of'"
             " declaration says '$partName'.");
         // The part is still included.
       }