Remove support for summary "fallback mode" from analyzer.

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 aeea3beb537331ed794f0cee3ec7bcf0301bbc20..53102b1200e031d3a975987b0871fc3c202859d8 100644
--- a/pkg/analyzer/lib/src/summary/idl.dart
+++ b/pkg/analyzer/lib/src/summary/idl.dart
@@ -634,6 +634,7 @@ abstract class LinkedLibrary extends base.SummaryClass {
    * Indicates whether this library was summarized in "fallback mode".  If
    * true, all other fields in the data structure have their default values.
    */
+  @deprecated
   @Id(5)
   bool get fallbackMode;
 
@@ -2868,6 +2869,7 @@ abstract class UnlinkedUnit extends base.SummaryClass {
    * When this field is non-empty, all other fields in the data structure have
    * their default values.
    */
+  @deprecated
   @Id(16)
   String get fallbackModePath;