Remove fallback mode and corresponding cli flag.

pkg/analyzer_cli/lib/src/build_mode.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer_cli.src.build_mode;
 
 import 'dart:core';
 import 'dart:io' as io;
 
 import 'package:analyzer/dart/ast/ast.dart' show CompilationUnit;
@@ skipping 174 matching lines @@
           break;
         }
       }
     }
 
     // Write summary.
     assembler = new PackageBundleAssembler(
         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();
       if (options.buildSummaryExcludeInformative) {
         bundle.flushInformative();
       }
       if (options.buildSummaryOutput != null) {
         io.File file = new io.File(options.buildSummaryOutput);
         file.writeAsBytesSync(bundle.toBuffer(), mode: io.FileMode.WRITE_ONLY);
       }
@@ skipping 202 matching lines @@
    * Build the inverse mapping of [uriToSourceMap].
    */
   static Map<String, Uri> _computePathToUriMap(Map<Uri, File> uriToSourceMap) {
     Map<String, Uri> pathToUriMap = <String, Uri>{};
     uriToSourceMap.forEach((Uri uri, File file) {
       pathToUriMap[file.path] = uri;
     });
     return pathToUriMap;
   }
 }