Add ContentCache.getExists() and use it in AnalysisContextImpl.exists().

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

 // Copyright (c) 2015, 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.
 
 library analyzer.src.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 792 matching lines @@
       dartWorkManager.addPriorityResult(target, RESOLVED_UNIT);
     }
     return null;
   }
 
   @override
   bool exists(Source source) {
     if (source == null) {
       return false;
     }
-    if (_contentCache.getContents(source) != null) {
-      return true;
+    bool overriddenExists = _contentCache.getExists(source);
+    if (overriddenExists != null) {

[Paul Berry, 2017/01/04 18:28:57]

Assuming you accept my proposal below, I think lines 813 through 817 can be
replaced with:

return _contentCache.getExists(source) || source.exists();

+      return overriddenExists;
     }
     return source.exists();
   }
 
   @override
   CacheEntry getCacheEntry(AnalysisTarget target) {
     CacheEntry entry = _cache.get(target);
     if (entry == null) {
       entry = new CacheEntry(target);
       ImplicitAnalysisEvent event = null;
@@ skipping 1437 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new ArgumentError(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }