Don't create FileState(s) for SDK source in _ContentCacheWrapper.

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/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart' show CompilationUnitElement;
@@ skipping 1531 matching lines @@
     throw new UnimplementedError();
   }
 
   @override
   String getContents(Source source) {
     return _getFileForSource(source).content;
   }
 
   @override
   bool getExists(Source source) {
+    if (source.isInSystemLibrary) {
+      return true;
+    }
     return _getFileForSource(source).exists;
   }
 
   @override
   int getModificationStamp(Source source) {
+    if (source.isInSystemLibrary) {
+      return 0;
+    }
     return _getFileForSource(source).exists ? 0 : -1;
   }
 
   @override
   String setContents(Source source, String contents) {
     throw new UnimplementedError();
   }
 
   FileState _getFileForSource(Source source) {
     String path = source.fullName;
@@ skipping 120 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }