Add the '--build-summary-only-ast' flag for generating summaries using only ASTs.

pkg/analyzer/lib/src/summary/package_bundle_reader.dart

 import 'dart:io' as io;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
@@ skipping 174 matching lines @@
 
   /**
    * Map from the URI of a library to the summary path that contained it.
    */
   final Map<String, String> uriToSummaryPath = <String, String>{};
 
   SummaryDataStore(Iterable<String> summaryPaths) {
     summaryPaths.forEach(_fillMaps);
   }
 
-  void _fillMaps(String path) {
-    io.File file = new io.File(path);
-    List<int> buffer = file.readAsBytesSync();
-    PackageBundle bundle = new PackageBundle.fromBuffer(buffer);
+  /**
+   * Add the given [bundle] loaded from the file with the given [path].
+   */
+  void addBundle(String path, PackageBundle bundle) {
     for (int i = 0; i < bundle.unlinkedUnitUris.length; i++) {
       String uri = bundle.unlinkedUnitUris[i];
       uriToSummaryPath[uri] = path;
       unlinkedMap[uri] = bundle.unlinkedUnits[i];
     }
     for (int i = 0; i < bundle.linkedLibraryUris.length; i++) {
       String uri = bundle.linkedLibraryUris[i];
       linkedMap[uri] = bundle.linkedLibraries[i];
     }
   }
+
+  void _fillMaps(String path) {
+    io.File file = new io.File(path);
+    List<int> buffer = file.readAsBytesSync();
+    PackageBundle bundle = new PackageBundle.fromBuffer(buffer);
+    addBundle(path, bundle);
+  }
 }
 
 /**
  * A concrete resynthesizer that serves summaries from given file paths.
  */
 class _FileBasedSummaryResynthesizer extends SummaryResynthesizer {
   final SummaryDataStore _dataStore;
 
   _FileBasedSummaryResynthesizer(
       SummaryResynthesizer parent,
@@ skipping 27 matching lines @@
  * also provides [summaryPath].
  */
 class _InSummaryFallbackSource extends FileBasedSource
     implements InSummarySource {
   @override
   final String summaryPath;
 
   _InSummaryFallbackSource(JavaFile file, Uri uri, this.summaryPath)
       : super(file, uri);
 }