Report CompileTimeErrorCode.DUPLICATE_PART when not only URI, but even Source is the same.

pkg/analyzer/test/generated/compile_time_error_code_test.dart

Index: pkg/analyzer/test/generated/compile_time_error_code_test.dart
diff --git a/pkg/analyzer/test/generated/compile_time_error_code_test.dart b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
index 927be4948078071571d1e0ff05c55e39e8d0d2f1..5fb1085bdb4138d2831188f1d52afcbbcabb6a0e 100644
--- a/pkg/analyzer/test/generated/compile_time_error_code_test.dart
+++ b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
@@ -1959,7 +1959,19 @@ main() {
     verify([source]);
   }
 
-  void test_duplicatePart() {
+  void test_duplicatePart_sameSource() {
+    addNamedSource('/part.dart', 'part of lib;');
+    Source source = addSource(r'''
+library lib;
+part 'part.dart';
+part 'foo/../part.dart';
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [CompileTimeErrorCode.DUPLICATE_PART]);
+    verify([source]);
+  }
+
+  void test_duplicatePart_sameUri() {
     addNamedSource('/part.dart', 'part of lib;');
     Source source = addSource(r'''
 library lib;