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

lib/src/checker/multi_package_resolver.dart

Index: lib/src/checker/multi_package_resolver.dart
diff --git a/lib/src/checker/multi_package_resolver.dart b/lib/src/checker/multi_package_resolver.dart
index bd95299856cdad40ee4ac0bcd7709c845534bbc8..d006b9841ac632215f90cbedaad28bb646677808 100644
--- a/lib/src/checker/multi_package_resolver.dart
+++ b/lib/src/checker/multi_package_resolver.dart
@@ -24,7 +24,12 @@ class MultiPackageResolver extends UriResolver {
     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));
   }