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

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

Index: pkg/analyzer/lib/src/summary/package_bundle_reader.dart
diff --git a/pkg/analyzer/lib/src/summary/package_bundle_reader.dart b/pkg/analyzer/lib/src/summary/package_bundle_reader.dart
index 878db38475fb12d6e2833a475be9943795c79877..59c195168f745c7c0dee7edf17f5bb45c5baa98b 100644
--- a/pkg/analyzer/lib/src/summary/package_bundle_reader.dart
+++ b/pkg/analyzer/lib/src/summary/package_bundle_reader.dart
@@ -192,10 +192,10 @@ class SummaryDataStore {
     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;
@@ -206,6 +206,13 @@ class SummaryDataStore {
       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);
+  }
 }
 
 /**