Add a fallback mode for building summaries.

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 028382531edf3787bec07d826be0800871bc7ed0..2003abf2dafa23fa4e405f98751f0e4a1672cabc 100644
--- a/pkg/analyzer_cli/lib/src/options.dart
+++ b/pkg/analyzer_cli/lib/src/options.dart
@@ -55,6 +55,9 @@ class CommandLineOptions {
   /// The path to output the summary when creating summaries in build mode.
   final String buildSummaryOutput;
 
+  /// Whether to output a summary in "fallback mode".
+  final bool buildSummaryFallback;
+
   /// Whether to suppress a nonzero exit code in build mode.
   final bool buildSuppressExitCode;
 
@@ -138,6 +141,7 @@ class CommandLineOptions {
       ArgResults args, Map<String, String> definedVariables)
       : buildAnalysisOutput = args['build-analysis-output'],
         buildMode = args['build-mode'],
+        buildSummaryFallback = args['build-summary-fallback'],
         buildSummaryInputs = args['build-summary-input'],
         buildSummaryOnly = args['build-summary-only'],
         buildSummaryOnlyDiet = args['build-summary-only-diet'],
@@ -356,6 +360,11 @@ 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,