This also changes the compiler so that we throw less often
on structural changes and produce a meaningful state
in the `ng.Program` in case of errors.
Related to #19951
PR Close#19953
For now, we always create all generated files, but diff them
before we pass them to TypeScript.
For the user files, we compare the programs and only emit changed
TypeScript files.
This also adds more diagnostic messages if the `—diagnostics` flag
is passed to the command line.
- don’t regenerate code for .d.ts files when
an oldProgram is passed to `createProgram`
- cache `fileExists` / `getSourceFile` / `readFile` in watch mode
- refactor tests to share common code in `test_support`
- support `—diagnostic` command line to print total time
used per watch mode compilation.
PR Close#19275
Chuck Jazdzewski
Tobias Bosch
Olivier Combe