Add a fallback mode for building summaries.

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

Index: pkg/analyzer/lib/src/summary/idl.dart
diff --git a/pkg/analyzer/lib/src/summary/idl.dart b/pkg/analyzer/lib/src/summary/idl.dart
index eb5eb7e091a0616550019fdc842fe4ca285f910f..a82ef1bf5fabfd4f405d25c8f87c172227c0c62e 100644
--- a/pkg/analyzer/lib/src/summary/idl.dart
+++ b/pkg/analyzer/lib/src/summary/idl.dart
@@ -437,6 +437,13 @@ abstract class LinkedLibrary extends base.SummaryClass {
   List<LinkedExportName> get exportNames;
 
   /**
+   * Indicates whether this library was summarized in "fallback mode".  If
+   * true, all other fields in the data structure have their default values.
+   */
+  @Id(5)
+  bool get fallbackMode;
+
+  /**
    * For each import in [UnlinkedUnit.imports], an index into [dependencies]
    * of the library being imported.
    */
@@ -2228,6 +2235,16 @@ abstract class UnlinkedUnit extends base.SummaryClass {
       generated.readUnlinkedUnit(buffer);
 
   /**
+   * If this compilation unit was summarized in fallback mode, the path where
+   * the compilation unit may be found on disk.  Otherwise empty.
+   *
+   * When this field is non-empty, all other fields in the data structure have
+   * their default values.
+   */
+  @Id(16)
+  String get fallbackModePath;
+
+  /**
    * Classes declared in the compilation unit.
    */
   @Id(2)