Bazel: Force sourcemaps inlining in polymer bundler

When enabled, create identity sourcemaps or use existing sourcemaps
during bundling. Each inline script will contain its own sourcemap.

Bug: Issue 9957
Change-Id: I2ed87d6a5047ed98aa00bff0b463e88098762fd5

tools/bzl/js.bzl

@@ -316,6 +316,7 @@ def _bundle_impl(ctx):
         "$p/" + ctx.file._bundler_archive.path,
         "--inline-scripts",
         "--inline-css",
+        "--sourcemaps",
         "--strip-comments",
         "--out-file",
         "$p/" + bundled.path,