Add a fallback mode for building summaries.

pkg/analyzer_cli/test/options_test.dart

Index: pkg/analyzer_cli/test/options_test.dart
diff --git a/pkg/analyzer_cli/test/options_test.dart b/pkg/analyzer_cli/test/options_test.dart
index 6e0f4cdf85e9ab711a3e2e97e5a9d243ba844bf5..63354da6fb99815861fa91ca259b5a258c833b2e 100644
--- a/pkg/analyzer_cli/test/options_test.dart
+++ b/pkg/analyzer_cli/test/options_test.dart
@@ -21,6 +21,7 @@ main() {
         expect(options, isNotNull);
         expect(options.buildMode, isFalse);
         expect(options.buildAnalysisOutput, isNull);
+        expect(options.buildSummaryFallback, isFalse);
         expect(options.buildSummaryInputs, isEmpty);
         expect(options.buildSummaryOnly, isFalse);
         expect(options.buildSummaryOutput, isNull);
@@ -250,6 +251,17 @@ class CommandLineOptionsTest extends AbstractStatusTest {
     expect(options.buildMode, isTrue);
   }
 
+  test_buildSummaryFallback() {
+    _parse([
+      '--build-mode',
+      '--build-summary-output=//path/to/output.sum',
+      '--build-summary-fallback',
+      'package:p/foo.dart|/path/to/p/lib/foo.dart'
+    ]);
+    expect(options.buildMode, isTrue);
+    expect(options.buildSummaryFallback, isTrue);
+  }
+
   test_buildSummaryInputs_commaSeparated() {
     _parse([
       '--build-mode',