Add an "api signature" to 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 60a0552f6e6048c4abd55fdd5bb052e9390fa8f6..15f81a7e69327eaede02330306a1439032699794 100644
--- a/pkg/analyzer/lib/src/summary/idl.dart
+++ b/pkg/analyzer/lib/src/summary/idl.dart
@@ -650,6 +650,14 @@ abstract class PackageBundle extends base.SummaryClass {
       generated.readPackageBundle(buffer);
 
   /**
+   * MD5 hash of the non-informative fields of the [PackageBundle] (not
+   * including this one).  This can be used to identify when the API of a
+   * package may have changed.
+   */
+  @Id(7)
+  String get apiSignature;
+
+  /**
    * Linked libraries.
    */
   @Id(0)