Throw a better exception for a modified analysis cache key

pkg/analysis_server/lib/src/domain_diagnostic.dart

Index: pkg/analysis_server/lib/src/domain_diagnostic.dart
diff --git a/pkg/analysis_server/lib/src/domain_diagnostic.dart b/pkg/analysis_server/lib/src/domain_diagnostic.dart
index e9bd0c3b1af263f461cc82dfe2ceba17704384ce..94b1c3a5eff46c2599fcc96914286425203392f3 100644
--- a/pkg/analysis_server/lib/src/domain_diagnostic.dart
+++ b/pkg/analysis_server/lib/src/domain_diagnostic.dart
@@ -63,6 +63,10 @@ class DiagnosticDomainHandler implements RequestHandler {
           AnalysisTarget target = iterator.key;
           if (countedTargets.add(target)) {
             CacheEntry cacheEntry = iterator.value;
+            if (cacheEntry == null) {
+              throw new StateError(
+                  "mutated cache key detected: $target (${target.runtimeType})");
+            }
             if (target is Source) {
               if (cacheEntry.explicitlyAdded) {
                 explicitFiles++;