Don't recompile dependency executables that haven't changed.

sdk/lib/_internal/pub/lib/src/source_registry.dart

Index: sdk/lib/_internal/pub/lib/src/source_registry.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source_registry.dart b/sdk/lib/_internal/pub/lib/src/source_registry.dart
index ffac4f8d7d8ac0bf43ba1b94b1b5ce005e36bbcc..d46ae93ae77299674c4b8004105cf8bf41bb7bf5 100644
--- a/sdk/lib/_internal/pub/lib/src/source_registry.dart
+++ b/sdk/lib/_internal/pub/lib/src/source_registry.dart
@@ -6,6 +6,7 @@ library pub.source_registry;
 
 import 'dart:collection';
 
+import 'package.dart';
 import 'source.dart';
 import 'source/unknown.dart';
 
@@ -24,6 +25,22 @@ class SourceRegistry extends IterableBase<Source> {
     return sources.iterator;
   }
 
+  /// Returns whether [id1] and [id2] refer to the same package, including
+  /// validating that their descriptions are equivalent.
+  bool idsEqual(PackageId id1, PackageId id2) {
+    if (id1 != id2) return false;
+    if (id1 == null && id2 == null) return true;
+    return idDescriptionsEqual(id1, id2);
+  }
+
+  /// Returns whether [id1] and [id2] have the same source and description.
+  ///
+  /// This doesn't check whether the name or versions are equal.
+  bool idDescriptionsEqual(PackageId id1, PackageId id2) {
+    if (id1.source != id2.source) return false;
+    return this[id1.source].descriptionsEqual(id1.description, id2.description);
+  }
+
   /// Sets the default source.
   ///
   /// This takes a string, which must be the name of a registered source.