Make pub build use barback.

sdk/lib/_internal/pub/lib/src/barback.dart

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index aba25de217427eb9eb6e5efc348534427bae24a8..a93d519e8104991b51c945b366d7c25dfdb8cdc6 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -184,3 +184,52 @@ AssetId specialUrlToId(Uri url) {
 
   return null;
 }
+
+/// Converts [id] to a "servable path" for that asset.
+///
+/// This is the relative URL that could be used to request that asset from pub
+/// serve. It's also the relative path that the asset will be output to by
+/// pub build (except this always returns a path using URL separators).
+///
+/// [entrypoint] is the name of the entrypoint package.
+///
+/// Examples (where [entrypoint] is "myapp"):
+///
+///     myapp|web/index.html   -> index.html
+///     myapp|lib/lib.dart     -> packages/myapp/lib.dart
+///     foo|lib/foo.dart       -> packages/foo/foo.dart
+///     foo|asset/foo.png      -> assets/foo/foo.png
+///     myapp|test/main.dart   -> ERROR
+///     foo|web/
+///
+/// Throws a [FormatException] if [id] is not a valid public asset.
+String idtoUrlPath(String entrypoint, AssetId id) {
+  var parts = path.url.split(id.path);
+
+  if (parts.length < 2) {
+    throw new FormatException(
+        "Can not serve asset from top-level directory.");
+  }
+
+  // Each top-level directory gets handled differently.
+  var dir = parts[0];
+  parts = parts.skip(1);
+
+  switch (dir) {
+    case "asset":
+      return path.url.join("assets", id.package, path.url.joinAll(parts));
+
+    case "lib":
+      return path.url.join("packages", id.package, path.url.joinAll(parts));
+
+    case "web":
+      if (id.package != entrypoint) {
+        throw new FormatException(
+            'Can only access "web" directory of root package.');
+      }
+      return path.url.joinAll(parts);
+
+    default:
+      throw new FormatException('Cannot access assets from "$dir".');
+  }
+}