Fix in multi-package-resolver to support files that will be created later (graph

lib/src/testing.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 dev_compiler.src.testing;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 251 matching lines @@
     return _parse(tokens[0]);
   }
 
   String toString() => '$level $type';
 }
 
 /// Uri resolver that can load test files from memory.
 class TestUriResolver extends UriResolver {
   final Map<Uri, TestSource> files = <Uri, TestSource>{};
 
-  TestUriResolver(Map<String, String> allFiles) {
+  /// Whether to represent a non-existing file with a [TestSource] (default
+  /// behavior from analyzer), or to use null (possible when overriding the
+  /// package-url-resolvers.)
+  final bool representNonExistingFiles;
+
+  TestUriResolver(Map<String, String> allFiles,
+      {this.representNonExistingFiles: true}) {
     allFiles.forEach((key, value) {
       var uri = key.startsWith('package:') ? Uri.parse(key) : new Uri.file(key);
       files[uri] = new TestSource(uri, value);
     });
   }
 
   Source resolveAbsolute(Uri uri) {
     if (uri.scheme != 'file' && uri.scheme != 'package') return null;
+    if (!representNonExistingFiles) return files[uri];
     return files.putIfAbsent(uri, () => new TestSource(uri, null));
   }
 }
 
 class TestContents implements TimestampedData<String> {
   int modificationTime;
   String data;
 
   TestContents(this.modificationTime, this.data);
 }
 
 /// An in memory source file.
 class TestSource implements Source {
   final Uri uri;
   TestContents contents;
   final SourceFile _file;
   final UriKind uriKind;
-  bool _exists;
 
   TestSource(uri, contents)
       : uri = uri,
-        _exists = contents != null,
         contents = new TestContents(1, contents),
         _file = contents != null ? new SourceFile(contents, url: uri) : null,
         uriKind = uri.scheme == 'file' ? UriKind.FILE_URI : UriKind.PACKAGE_URI;
 
-  bool exists() => _exists;
+  bool exists() => contents.data != null;
 
   Source get source => this;
 
   String _encoding;
   String get encoding => _encoding != null ? _encoding : (_encoding = '$uri');
 
   String get fullName => uri.path;
 
   int get modificationStamp => contents.modificationTime;
   String get shortName => path.basename(uri.path);
 
   operator ==(other) => other is TestSource && uri == other.uri;
   int get hashCode => uri.hashCode;
   bool get isInSystemLibrary => false;
 
   Uri resolveRelativeUri(Uri relativeUri) => uri.resolveUri(relativeUri);
 
   SourceSpan spanFor(AstNode node) {
     final begin = node is AnnotatedNode
         ? node.firstTokenAfterCommentAndMetadata.offset
         : node.offset;
     return _file.span(begin, node.end);
   }
 
   String toString() => '[$runtimeType: $uri]';
 }