perf(@ngtools/webpack): avoid bootstrap conversion AST traversal where possible
perf(@ngtools/webpack): avoid bootstrap conversion AST traversal where possible

To support AOT compilation the Angular bootstrap call needs to be converted from
using `platform-browser-dynamic` to `platform-browser`. This transform was previously
being executed against every source file within the program regardless of the presence of
`platformBrowserDynamic`. An initial check is now performed that can avoid AST traversal
when the bootstrapping call is not present in a file.

angular/angular-cli,clydin/angular-cli,angular/angular-cli,angular/angular-cli,angular/angular-cli,clydin/angular-cli,clydin/angular-cli,clydin/angular-cli