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

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index d15c36f4d1c243c67ebfc7f0a2fb040d2102e0fb..25bff4608e1a444521557af16007d6f74fd44632 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -1510,7 +1510,7 @@ class BuildLibraryElementTask extends SourceBasedAnalysisTask {
     LibraryIdentifier libraryNameNode = null;
     String partsLibraryName = _UNKNOWN_LIBRARY_NAME;
     bool hasPartDirective = false;
-    Set<String> seenPartUris = new Set<String>();
+    Set<Source> seenPartSources = new Set<Source>();
     FunctionElement entryPoint =
         _findEntryPoint(definingCompilationUnitElement);
     List<Directive> directivesToResolve = <Directive>[];
@@ -1534,9 +1534,9 @@ class BuildLibraryElementTask extends SourceBasedAnalysisTask {
           partElement.uriEnd = partUri.end;
           partElement.uri = directive.uriContent;
           //
-          // Validate that the part URI is unique in the library.
+          // Validate that the part source is unique in the library.
           //
-          if (!seenPartUris.add(directive.uriContent)) {
+          if (!seenPartSources.add(partSource)) {
             errors.add(new AnalysisError(
                 librarySource,
                 partUri.offset,