Use modifiable top-level scope for Fasta parser in file_state.

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 b1dc47445a9add3403d2e5cf152b44d96bfacafe..acca51444046871ee54c34084d1f6a24b223deb1 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(),
-            null,
+            new fasta.Scope.top(isModifiable: true),
             uri);
         var parser = new fasta.Parser(astBuilder);
         parser.parseUnit(scanResult.tokens);