Don't add files inside "packages" subdir to context.

pkg/analysis_server/lib/src/resource.dart

Index: pkg/analysis_server/lib/src/resource.dart
diff --git a/pkg/analysis_server/lib/src/resource.dart b/pkg/analysis_server/lib/src/resource.dart
index b8b6c3cd6523985cd0ed72ade8bb978d42c92793..e3df8afb24bb5e0814f48220e45d964d8ee132ac 100644
--- a/pkg/analysis_server/lib/src/resource.dart
+++ b/pkg/analysis_server/lib/src/resource.dart
@@ -80,6 +80,11 @@ abstract class ResourceProvider {
    * Return the [Resource] that corresponds to the given [path].
    */
   Resource getResource(String path);
+
+  /**
+   * Get the path context used by this resource provider.
+   */
+  Context get pathContext;
 }
 
 
@@ -347,6 +352,9 @@ class MemoryResourceProvider implements ResourceProvider {
     _pathToTimestamp.remove(path);
     _notifyWatchers(path, ChangeType.REMOVE);
   }
+
+  @override
+  Context get pathContext => posix;
 }
 
 
@@ -442,4 +450,7 @@ class PhysicalResourceProvider implements ResourceProvider {
       return new _PhysicalFile(file);
     }
   }
+
+  @override
+  Context get pathContext => io.Platform.isWindows ? windows : posix;
 }