Previously, `DeferredRewriter` checked for the existence of
`.ng_deps.dart` files to determine which deferred libraries it needed to
rewrite, requiring that those assets exist at the time it was run.
Update to check for `.ng_meta.json` files instead, which exist after the
`DirectiveProcessor` phase. This allows the `DeferredRewriter` (which
only processes *.dart files) to run in
parallel with `TemplateComplier` (which only processes *.ng_meta.json
files) and `StylesheetCompiler` (which only processes *.css files).
Have DeferredRewriter to check existence of .ng_meta.json assets rather than .ng_deps.dart assets
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