Bring back initPolymer, allow boot.js only if using "polymer_experimental.html".

pkg/code_transformers/lib/assets.dart

Index: pkg/code_transformers/lib/assets.dart
diff --git a/pkg/code_transformers/lib/assets.dart b/pkg/code_transformers/lib/assets.dart
index 4bafc268c6c48623ed9bab4c467c807234053492..9a76c876651476da3bd25a150d48a75575f0941c 100644
--- a/pkg/code_transformers/lib/assets.dart
+++ b/pkg/code_transformers/lib/assets.dart
@@ -64,6 +64,8 @@ AssetId uriToAssetId(AssetId source, String url, TransformLogger logger,
       // web/, test/, example/, etc) are resolved as an asset in another
       // package. 'packages' can be used anywhere, there is no need to walk up
       // where the entrypoint file was.
+      // TODO(sigmund): this needs to change: Only resolve when index == 1 &&
+      // topFolder == segment[0], otherwise give a warning (dartbug.com/17596).
       return _extractOtherPackageId(index, segments, logger, span);
     } else if (index == 1 && segments[0] == '..') {
       // Relative URLs of the form "../../packages/foo/bar" in an asset under