Actually limit number of stored exception contexts.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:typed_data';
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1146 matching lines @@
       exception = state.exception;
       stackTrace = state.stackTrace;
       contextKey = state.contextKey;
     }
     CaughtException caught = new CaughtException(exception, stackTrace);
     _exceptionController.add(new ExceptionResult(path, caught, contextKey));
   }
 
   String _storeExceptionContext(
       String path, FileState libraryFile, exception, StackTrace stackTrace) {
-    if (allowedNumberOfContextsToWrite > 0) {
+    if (allowedNumberOfContextsToWrite <= 0) {
+      return null;
+    } else {
       allowedNumberOfContextsToWrite--;
     }
     try {
       List<AnalysisDriverExceptionFileBuilder> contextFiles = libraryFile
           .transitiveFiles
           .map((file) => new AnalysisDriverExceptionFileBuilder(
               path: file.path, content: file.content))
           .toList();
       contextFiles.sort((a, b) => a.path.compareTo(b.path));
       AnalysisDriverExceptionContextBuilder contextBuilder =
@@ skipping 691 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }