Use Set instead of List for 'dependentResults'.

pkg/analyzer/lib/src/context/cache.dart

Index: pkg/analyzer/lib/src/context/cache.dart
diff --git a/pkg/analyzer/lib/src/context/cache.dart b/pkg/analyzer/lib/src/context/cache.dart
index 35ed5568642616ce04a626ff9c0a09001b13b611..ce0bf990244d4f386e2d98a65123220aeed6d2cd 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -467,8 +467,8 @@ class CacheEntry {
       }
     });
     // Invalidate results that depend on this result.
-    List<TargetedResult> dependentResults = thisData.dependentResults;
-    thisData.dependentResults = <TargetedResult>[];
+    Set<TargetedResult> dependentResults = thisData.dependentResults;
+    thisData.dependentResults = new Set<TargetedResult>();
     dependentResults.forEach((TargetedResult dependentResult) {
       CacheEntry entry = _partition.get(dependentResult.target);
       if (entry != null) {
@@ -935,7 +935,7 @@ class ResultData {
   /**
    * A list of the results that depend on this result.
    */
-  List<TargetedResult> dependentResults = <TargetedResult>[];
+  Set<TargetedResult> dependentResults = new Set<TargetedResult>();
 
   /**
    * Initialize a newly created result holder to represent the value of data