Add BazelWorkspace and move BazelFileUriResolver to it.

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index f005eefd8c90a9d2b367c3b0e63ebb0769da0d32..ac8c0db6652a0ee187ba92c9640b1f2294cbb1f3 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -260,8 +260,8 @@ class ResourceUriResolver extends UriResolver {
     if (!isFileUri(uri)) {
       return null;
     }
-    Resource resource =
-        _provider.getResource(_provider.pathContext.fromUri(uri));
+    String path = _provider.pathContext.fromUri(uri);
+    Resource resource = _provider.getResource(path);
     if (resource is File) {
       return resource.createSource(actualUri ?? uri);
     }