Remove fallback mode and corresponding cli flag.

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 d34d7afdd8cfff71b10c7d6a5acd46213d0efe59..b74edc16fd241c2770f593219fb89c5731bb070a 100644
--- a/pkg/analyzer_cli/test/options_test.dart
+++ b/pkg/analyzer_cli/test/options_test.dart
@@ -21,7 +21,6 @@ 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);
@@ -264,17 +263,6 @@ class CommandLineOptionsTest extends AbstractStatusTest {
     expect(options.sourceFiles, isEmpty);
   }
 
-  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',