Make summary format stable when adding getters to idl.

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

Index: pkg/analyzer/lib/src/summary/base.dart
diff --git a/pkg/analyzer/lib/src/summary/base.dart b/pkg/analyzer/lib/src/summary/base.dart
index a04476bcab54e72ff300c47afdff564dd138bdbe..c4a4cdb8a6aeac9b622bd2099f44e0842a69574a 100644
--- a/pkg/analyzer/lib/src/summary/base.dart
+++ b/pkg/analyzer/lib/src/summary/base.dart
@@ -8,6 +8,21 @@
 library analyzer.src.summary.base;
 
 /**
+ * Annotation used in the summary IDL to indicate the id of a field.  The set
+ * of ids used by a class must cover the contiguous range from 0 to N-1, where
+ * N is the number of fields.
+ *
+ * In order to preserve forwards and backwards compatibility, id numbers must
+ * be stable between releases.  So when new fields are added they should take
+ * the next available id without renumbering other fields.
+ */
+class Id {
+  final int value;
+
+  const Id(this.value);
+}
+
+/**
  * Instances of this class represent data that has been read from a summary.
  */
 abstract class SummaryClass {