Return empty contents for not existing sources.

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/src/cancelable_future.dart';
@@ skipping 1007 matching lines @@
   bool isServerLibrary(Source librarySource) {
     CacheEntry entry = _cache.get(librarySource);
     return !entry.getValue(IS_CLIENT) && entry.getValue(IS_LAUNCHABLE);
   }
 
   @override
   CompilationUnit parseCompilationUnit(Source source) {
     if (!AnalysisEngine.isDartFileName(source.shortName)) {
       return null;
     }
+    try {
+      getContents(source);
+    } catch (exception, stackTrace) {
+      throw new AnalysisException('Could not get contents of $source',
+          new CaughtException(exception, stackTrace));
+    }
     return _computeResult(source, PARSED_UNIT);
   }
 
   @override
   ht.HtmlUnit parseHtmlUnit(Source source) {
     if (!AnalysisEngine.isHtmlFileName(source.shortName)) {
       return null;
     }
     // TODO(brianwilkerson) Implement HTML analysis.
     return null; //_computeResult(source, null);
@@ skipping 990 matching lines @@
     PendingFuture pendingFuture =
         new PendingFuture<T>(_context, target, computeValue);
     if (!pendingFuture.evaluate(entry)) {
       _context._pendingFutureTargets
           .putIfAbsent(target, () => <PendingFuture>[])
           .add(pendingFuture);
     }
     return pendingFuture.future;
   }
 }