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

sdk/lib/_internal/pub/test/snapshot/doesnt_resnapshot_when_a_dependency_is_unchanged_test.dart

Index: sdk/lib/_internal/pub/test/snapshot/doesnt_resnapshot_when_a_dependency_is_unchanged_test.dart
diff --git a/sdk/lib/_internal/pub/test/snapshot/doesnt_snapshot_transitive_dependencies_test.dart b/sdk/lib/_internal/pub/test/snapshot/doesnt_resnapshot_when_a_dependency_is_unchanged_test.dart
similarity index 57%
copy from sdk/lib/_internal/pub/test/snapshot/doesnt_snapshot_transitive_dependencies_test.dart
copy to sdk/lib/_internal/pub/test/snapshot/doesnt_resnapshot_when_a_dependency_is_unchanged_test.dart
index 9b4512e7f9666be7ea99f86a86a4e129829e725a..c4ab2f08266ef29d446db073df7571aafb9ee08d 100644
--- a/sdk/lib/_internal/pub/test/snapshot/doesnt_snapshot_transitive_dependencies_test.dart
+++ b/sdk/lib/_internal/pub/test/snapshot/doesnt_resnapshot_when_a_dependency_is_unchanged_test.dart
@@ -12,10 +12,10 @@ import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("doesn't create a snapshot for transitive dependencies' "
-      "executables", () {
+  integration("doesn't recreate a snapshot when no dependencies of a package "
+      "have changed", () {
     servePackages([
-      packageMap("foo", "1.2.3", {'bar': '1.2.3'}),
+      packageMap("foo", "1.2.3", {"bar": "any"}),
       packageMap("bar", "1.2.3")
     ], contents: [
       d.dir("bin", [d.file("hello.dart", "void main() => print('hello!');")])
@@ -23,8 +23,13 @@ main() {
 
     d.appDir({"foo": "1.2.3"}).create();
 
-    pubGet();
+    pubGet(output: contains("Precompiled foo:hello."));
 
-    d.nothing(p.join(appPath, '.pub', 'bin', 'bar')).validate();
+    pubUpgrade(output: isNot(contains("Precompiled foo:hello.")));
+
+    d.dir(p.join(appPath, '.pub', 'bin'), [
+      d.file('sdk-version', '0.1.2+3\n'),
+      d.dir('foo', [d.matcherFile('hello.dart.snapshot', contains('hello!'))])
+    ]).validate();
   });
 }