Show the dependender(s) for missing packages.

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

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 222941ad9f2d012f285f614f0ab82004d85a3d6a..66d12204a84485377745d39d4112fc4d59d8e77b 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -804,6 +804,16 @@ class ApplicationException implements Exception {
   String toString() => message;
 }
 
+/// An class for exceptions where a package could not be found in a [Source].
+///
+/// The source is responsible for wrapping its internal exceptions in this so
+/// that other code in pub can use this to show a more detailed explanation of
+/// why the package was being requested.
+class PackageNotFoundException extends ApplicationException {
+  PackageNotFoundException(String message, [innerError, StackTrace innerTrace])
+      : super(message, innerError, innerTrace);
+}
+
 /// Throw a [ApplicationException] with [message].
 void fail(String message, [innerError, StackTrace innerTrace]) {
   throw new ApplicationException(message, innerError, innerTrace);