Remove fallback mode and corresponding cli flag.

pkg/analyzer_cli/lib/src/build_mode.dart

Index: pkg/analyzer_cli/lib/src/build_mode.dart
diff --git a/pkg/analyzer_cli/lib/src/build_mode.dart b/pkg/analyzer_cli/lib/src/build_mode.dart
index a1aa4384d3da98a548bfdaf12bc38f57cdd8b89c..8992a10dcb228c69b4ae4e031bd280d770494eb1 100644
--- a/pkg/analyzer_cli/lib/src/build_mode.dart
+++ b/pkg/analyzer_cli/lib/src/build_mode.dart
@@ -192,13 +192,7 @@ class BuildMode {
         excludeHashes: options.buildSummaryExcludeInformative &&
             options.buildSummaryOutputSemantic == null);
     if (_shouldOutputSummary) {
-      if (!options.buildSummaryFallback) {
-        _serializeAstBasedSummary(explicitSources);
-      } else {
-        for (Source source in explicitSources) {
-          assembler.addFallbackUnit(source);
-        }
-      }
+      _serializeAstBasedSummary(explicitSources);
       // Write the whole package bundle.
       assembler.recordDependencies(summaryDataStore);
       PackageBundleBuilder bundle = assembler.assemble();