Capture more errors and improve duplicate error removal

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

Index: pkg/analyzer/lib/src/generated/error.dart
diff --git a/pkg/analyzer/lib/src/generated/error.dart b/pkg/analyzer/lib/src/generated/error.dart
index 3ff91a83528543a73c64022ca2593aab057337df..0881a7c63091762e62706e466edf27c076c4655e 100644
--- a/pkg/analyzer/lib/src/generated/error.dart
+++ b/pkg/analyzer/lib/src/generated/error.dart
@@ -209,11 +209,11 @@ class AnalysisError {
    * a single list of errors.
    */
   static List<AnalysisError> mergeLists(List<List<AnalysisError>> errorLists) {
-    List<AnalysisError> errors = <AnalysisError>[];
+    Set<AnalysisError> errors = new HashSet<AnalysisError>();
     for (List<AnalysisError> errorList in errorLists) {
       errors.addAll(errorList);
     }
-    return errors;
+    return errors.toList();
   }
 }