Fix for 'Create part' Quick Fix.

pkg/analyzer/lib/source/package_map_resolver.dart

Index: pkg/analyzer/lib/source/package_map_resolver.dart
diff --git a/pkg/analyzer/lib/source/package_map_resolver.dart b/pkg/analyzer/lib/source/package_map_resolver.dart
index 58240c58bd1d6ddde05fa3e12ce50fdc85cf4350..1b0f82fde19b6a78277d3186458b253ca572b043 100644
--- a/pkg/analyzer/lib/source/package_map_resolver.dart
+++ b/pkg/analyzer/lib/source/package_map_resolver.dart
@@ -71,7 +71,10 @@ class PackageMapUriResolver extends UriResolver {
     // Return a NonExistingSource instance.
     // This helps provide more meaningful error messages to users
     // (a missing file error, as opposed to an invalid URI error).
-    return new NonExistingSource(uri.toString(), UriKind.PACKAGE_URI);
+    String fullPath = packageDirs != null && packageDirs.isNotEmpty
+        ? packageDirs.first.canonicalizePath(relPath)
+        : relPath;
+    return new NonExistingSource(fullPath, uri, UriKind.PACKAGE_URI);
   }
 
   @override