- Properly return null if an unknown package is resolved.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 9a34b51cb0b4c31a7ae682b7637966357b81c124..2f64b77be680e432168a986bfa4da67f48d16b48 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -295,7 +295,18 @@ Uri _resolvePackageUri(Uri uri) {
     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) {
@@ -754,7 +765,17 @@ Future<Uri> _resolvePackageUriFuture(Uri packageUri) async {
   }
   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'");
   }