Remove fallback mode and corresponding cli flag.

pkg/analyzer_cli/lib/src/options.dart

Index: pkg/analyzer_cli/lib/src/options.dart
diff --git a/pkg/analyzer_cli/lib/src/options.dart b/pkg/analyzer_cli/lib/src/options.dart
index 3061c5ce1954cd0b0c41dc9ba3bd35067629ce1c..70da5919af7a3f1fa38a581bac177b5c19d52bc2 100644
--- a/pkg/analyzer_cli/lib/src/options.dart
+++ b/pkg/analyzer_cli/lib/src/options.dart
@@ -62,9 +62,6 @@ class CommandLineOptions {
   /// build mode.
   final String buildSummaryOutputSemantic;
 
-  /// Whether to output a summary in "fallback mode".
-  final bool buildSummaryFallback;
-
   /// Whether to suppress a nonzero exit code in build mode.
   final bool buildSuppressExitCode;
 
@@ -165,7 +162,6 @@ class CommandLineOptions {
       : buildAnalysisOutput = args['build-analysis-output'],
         buildMode = args['build-mode'],
         buildModePersistentWorker = args['persistent_worker'],
-        buildSummaryFallback = args['build-summary-fallback'],
         buildSummaryInputs = args['build-summary-input'] as List<String>,
         buildSummaryOnly = args['build-summary-only'],
         buildSummaryOnlyDiet = args['build-summary-only-diet'],
@@ -426,11 +422,6 @@ class CommandLineOptions {
           defaultsTo: false,
           negatable: false,
           hide: true)
-      ..addFlag('build-summary-fallback',
-          help: 'If outputting a summary, output it in fallback mode.',
-          defaultsTo: false,
-          negatable: false,
-          hide: true)
       ..addFlag('build-suppress-exit-code',
           help: 'Exit with code 0 even if errors are found.',
           defaultsTo: false,