Make top-level scope implicit.

pkg/analyzer/lib/src/dart/analysis/file_state.dart

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 1cbd5deac27e0b1043405a4a42f61e89df8f6b69..b1dc47445a9add3403d2e5cf152b44d96bfacafe 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -364,7 +364,7 @@ class FileState {
             null,
             null,
             new _FastaElementStoreProxy(),
-            new _FastaEmptyScope(),
+            null,
             uri);
         var parser = new fasta.Parser(astBuilder);
         parser.parseUnit(scanResult.tokens);
@@ -801,10 +801,6 @@ class _FastaElementStoreProxy implements fasta.ElementStore {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
-class _FastaEmptyScope extends fasta.Scope {
-  _FastaEmptyScope() : super({}, null);
-}
-
 class _FastaInterfaceTypeProxy implements fasta.KernelInterfaceType {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }