Actually limit number of stored exception contexts.

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 7e923024015fde3f256eb2ea3ed6fdd776a37e52..46a4056b7eae0cbfb882e9fbb2eb913057c62d50 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -1164,7 +1164,9 @@ class AnalysisDriver {
 
   String _storeExceptionContext(
       String path, FileState libraryFile, exception, StackTrace stackTrace) {
-    if (allowedNumberOfContextsToWrite > 0) {
+    if (allowedNumberOfContextsToWrite <= 0) {
+      return null;
+    } else {
       allowedNumberOfContextsToWrite--;
     }
     try {