- Properly return null if an unknown package is resolved.

runtime/bin/builtin.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library builtin;
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_internal';
 import 'dart:isolate';
@@ skipping 277 matching lines @@
     resolvedUri = _packageRoot.resolve(uri.path);
   } else {
     var packageName = uri.pathSegments[0];
     var mapping = _packageMap[packageName];
     if (_traceLoading) {
       _log("Mapped '$packageName' package to '$mapping'");
     }
     if (mapping == null) {
       throw "No mapping for '$packageName' package when resolving '$uri'.";
     }
-    var path = uri.path.substring(packageName.length + 1);
+    var path;
+    if (uri.path.length > packageName.length) {
+      path = uri.path.substring(packageName.length + 1);
+    } else {
+      // Handle naked package resolution to the default package name:
+      // package:foo is equivalent to package:foo/foo.dart
+      assert(uri.path.length == packageName.length);
+      path = "$packageName.dart";

[Lasse Reichstein Nielsen, 2016/03/23 11:18:18]

While I like this change in general, it should be supported by all other
implementations as well, otherwise you can have code that works in the VM but
not in dart2js. Is that planned as well?

Otherwise, how about just resolving the path to "/"?

+    }
+    if (_traceLoading) {
+      _log("Path to be resolved in package: $path");
+    }
     resolvedUri = mapping.resolve(path);
   }
   if (_traceLoading) {
     _log("Resolved '$uri' to '$resolvedUri'.");
   }
   return resolvedUri;
 }
 
 
 // Resolves the script uri in the current working directory iff the given uri
@@ skipping 438 matching lines @@
 
   if (!_packagesReady) {
     if (_traceLoading) {
       _log("Trigger loading by requesting the package config.");
     }
     // Make sure to trigger package resolution.
     var dummy = await _getPackageConfigFuture();
   }
   assert(_packagesReady);
 
-  var result = _resolvePackageUri(packageUri);
+  var result;
+  try {
+    result = _resolvePackageUri(packageUri);
+  } catch (e, s) {
+    // Any error during resolution will resolve this package as not mapped,
+    // which is indicated by a null return.
+    if (_traceLoading) {
+      _log("Exception ($e) when resolving package URI: $packageUri");
+    }
+    result = null;
+  }
   if (_traceLoading) {
     _log("Resolved '$packageUri' to '$result'");
   }
   return result;
 }
 
 
 // Handling of Resource class by dispatching to the load port.
 Future<List<int>> _resourceReadAsBytes(Uri uri) {
   var completer = new Completer<List<int>>();
@@ skipping 119 matching lines @@
 
 // Register callbacks and hooks with the rest of the core libraries.
 _setupHooks() {
   _setupCompleted = true;
   VMLibraryHooks.resourceReadAsBytes = _resourceReadAsBytes;
 
   VMLibraryHooks.packageRootUriFuture = _getPackageRootFuture;
   VMLibraryHooks.packageConfigUriFuture = _getPackageConfigFuture;
   VMLibraryHooks.resolvePackageUriFuture = _resolvePackageUriFuture;
 }