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

lib/src/checker/multi_package_resolver.dart

 // Copyright (c) 2014, 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.checker.multi_package_resolver;
 
 import 'dart:io';
 
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:path/path.dart' show join;
 
 /// A package resolver that supports a non-standard package layout, where
 /// packages with dotted names are expanded to a hierarchy of directories, and
 /// packages can be found on one or more locations.
 class MultiPackageResolver extends UriResolver {
   final List<String> searchPaths;
   MultiPackageResolver(this.searchPaths);
 
   @override
   Source resolveAbsolute(Uri uri) {
     var path = _expandPath(uri);
     if (path == null) return null;
 
     var resolvedPath = _resolve(path);
-    if (resolvedPath == null) return null;
+    if (resolvedPath == null) {
+      // If we can't resolve it, we default to use the path with the first
+      // prefix. The returned source will have that `source.exists()` is false.

[vsm, 2015/03/11 20:07:50]

Does this mean that the directory for generated code must be first on the
searchPath?

Perhaps it's better to make it a separate arg then?

[Siggi Cherem (dart-lang), 2015/03/12 18:26:40]

After this and our discussion offline, I decided to take a second shot at this.
Instead of picking one default, I'm allowing to keep the source as null in other
parts of the system, and recover from it later.

+      var prefix = searchPaths.isEmpty ? '' : searchPaths[0];
+      resolvedPath = join(prefix, path);
+    }
 
     return new FileBasedSource.con2(uri, new JavaFile(resolvedPath));
   }
 
   /// Resolve [path] by looking at each prefix in [searchPaths] and returning
   /// the first location where `prefix + path` exists.
   String _resolve(String path) {
     for (var prefix in searchPaths) {
       var resolvedPath = join(prefix, path);
       if (new File(resolvedPath).existsSync()) return resolvedPath;
     }
     return null;
   }
 
   /// Expand `uri.path`, replacing dots in the package name with slashes.
   String _expandPath(Uri uri) {
     if (uri.scheme != 'package') return null;
     var path = uri.path;
     var slashPos = path.indexOf('/');
     var packagePath = path.substring(0, slashPos).replaceAll(".", "/");
     var filePath = path.substring(slashPos + 1);
     return '${packagePath}/lib/${filePath}';
   }
 }