Add implementation of Isolate.resolvePackageUri for dart2js.

sdk/lib/_internal/js_runtime/lib/isolate_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart b/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
index ed72e6c09fa64493de08b5803cab910249f3cedf..8ba178d5fb62ff6729643bf55c56006b86df9453 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_patch.dart
@@ -32,8 +32,9 @@ class Isolate {
   }
 
   @patch
-  static Future<Uri> resolvePackageUri(Uri packageUri) {
-    throw new UnsupportedError("Isolate.resolvePackageUri");
+  static Future<Uri> resolvePackageUri(Uri packageUri) async {
+    var base = IsolateNatives.packagesBase;
+    return Uri.base.resolve('$base/${packageUri.path}');

[Lasse Reichstein Nielsen, 2016/11/14 20:42:34]

There is no validation of the packageUri.
If the argument isn't a package: URI, it must be returned unchanged
So, start with:
  if (packageUri.scheme != "package") return packageUri;

For the resolution, I think it will work, but it would be better if you could
retain any fragment. It makes a kind of sense to be able to do:
Isolate.resolvePackageUri(Uri.parse("package:foo/asset/foo.zip#file.ext"))
(or something similar).

Maybe:
  return Uri.base.resolve(
      '$base/${packageUri.toString().subString("package:".length)}');

(I'm sure that can be made more efficient, but it's a good start).

That said, I'm not sure the VM does that either.

[Lasse Reichstein Nielsen, 2016/11/14 20:46:23]

Actually, it can probably just be:
  Uri.base.resolve("$base/").resolveUri(packageUri.replace(scheme: ""))
(and then it's probably worth it to cache Uri.base.resolve("$base/") instead of
computing it each time).

[Siggi Cherem (dart-lang), 2016/11/14 23:07:15]

Done. I'm checking for the package scheme first, then using this approach.

[Siggi Cherem (dart-lang), 2016/11/14 23:07:15]

Done.

   }
 
   @patch