Add a fallback mode for building summaries.

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 0c20851589ee8c43d5becac6fd2fab4e8a82428c..42c3dc1c6e430158ea4a794391d3bf2bf58aa39a 100644
--- a/pkg/analyzer_cli/lib/src/build_mode.dart
+++ b/pkg/analyzer_cli/lib/src/build_mode.dart
@@ -86,11 +86,17 @@ class BuildMode {
     if (options.buildSummaryOutput != null) {
       PackageBundleAssembler assembler = new PackageBundleAssembler();
       for (Source source in explicitSources) {
-        if (context.computeKindOf(source) != SourceKind.LIBRARY) {
-          continue;
+        if (context.computeKindOf(source) == SourceKind.LIBRARY) {
+          if (options.buildSummaryFallback) {
+            assembler.addFallbackLibrary(source);
+          } else {
+            LibraryElement libraryElement = context.computeLibraryElement(source);
+            assembler.serializeLibraryElement(libraryElement);
+          }
+        }
+        if (options.buildSummaryFallback) {
+          assembler.addFallbackUnit(source);
         }
-        LibraryElement libraryElement = context.computeLibraryElement(source);
-        assembler.serializeLibraryElement(libraryElement);
       }
       // Write the whole package bundle.
       PackageBundleBuilder sdkBundle = assembler.assemble();