Avoid internalizing on-the-fly now that scope analysis and natives syntax
runtime calls no longer require internalized AST values. This should be
more efficient by avoiding extra branches on every AST value creation.

BUG=v8:5215, chromium:634953

Review-Url: https://codereview.chromium.org/2328593002
Cr-Commit-Position: refs/heads/master@{#39531}