Properly report assets from static packages as not found.

sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart

Index: sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
diff --git a/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart b/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
index f3769eed49862342b5d090a3d53830a179609fbf..613d9cbd99099a608bd6e98180b76344632abc3a 100644
--- a/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
+++ b/sdk/lib/_internal/pub/test/get/cache_transformed_dependency_test.dart
@@ -32,6 +32,30 @@ class ModeTransformer extends Transformer {
 }
 """;
 
+const HAS_INPUT_TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class HasInputTransformer extends Transformer {
+  HasInputTransformer.asPlugin();
+
+  bool get allowedExtensions => '.txt';
+
+  Future apply(Transform transform) {
+    return Future.wait([
+      transform.hasInput(new AssetId("foo", "lib/foo.dart")),
+      transform.hasInput(new AssetId("foo", "lib/does/not/exist.dart"))
+    ]).then((results) {
+      transform.addOutput(new Asset.fromString(
+          transform.primaryInput.id,
+          "lib/foo.dart: \${results.first}, "
+              "lib/does/not/exist.dart: \${results.last}"));
+    });
+  }
+}
+""";
+
 main() {
   initConfig();
 
@@ -314,6 +338,44 @@ main() {
     pub.stdout.expect("Hello!");
     pub.shouldExit();
   });
+
+  // Regression test for issue 21087.
+  integration("hasInput works for static packages", () {
+    servePackages((builder) {
+      builder.serveRepoPackage('barback');
+
+      builder.serve("foo", "1.2.3",
+          deps: {'barback': 'any'},
+          pubspec: {'transformers': ['foo']},
+          contents: [
+        d.dir("lib", [
+          d.file("transformer.dart", replaceTransformer("Hello", "Goodbye")),
+          d.file("foo.dart", "void main() => print('Hello!');")
+        ])
+      ]);
+    });
+
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "dependencies": {"foo": "1.2.3"},
+        "transformers": ["myapp/src/transformer"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", HAS_INPUT_TRANSFORMER)
+      ])]),
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
+    ]).create();
+
+    pubGet(output: contains("Precompiled foo."));
+
+    pubServe();
+    requestShouldSucceed("foo.txt",
+        "lib/foo.dart: true, lib/does/not/exist.dart: false");
+    endPubServe();
+  });
 }
 
 String replaceTransformer(String input, String output) {