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We do this by changing three things:

1. Set `composite` to `true`. From Typescript docs:
```
The composite option enforces certain constraints which make it possible
for build tools to quickly determine if a project has been built yet.
```
This results in the TypeScript compiler generating two new files for
each mojom target:
  - Declaration files (.d.ts) which in the Mojo case describes the
    exported Mojo interface.
  - tsBuildInfo files (.tsbuildinfo) which store incremental
    compilation information as a part of composite projects which
    enables faster building of larger TypeScript codebases.

2. Adding `references` to tsconfigs, to tell the compile that a target
   depends on other targets.

3. Adding `/// <reference>` to non-es-modules files, to tell the
   compiler which files that file references. Somewhat surprisingly
   adding `references` to the tsconfig was not enough.

This results in:

1. Each mojom target getting compiled only once, independent of how many
   other targets depend on it.
2. The compiler complaining if we forget to add a mojom target to
   `references` and if we forget to add `/// <reference>`s for files
   in non-es-modules files.
3. When compiling a mojom target, the compiler only attempts to compile
   the specific target i.e. it doesn't try to compile its
   dependencies. It's up to ninja to trigger the compilation of
   dependencies. If a dependent target hasn't been compiled yet,
   the compiler will throw an error. This ensures that we don't
   accidentally forget dependencies in our generated bindings.

The upside of this, is that compilation order is controlled by gn
and ninja. The downside is that we need to run an instance of the
compiler for each target and we don't get to take advantage of the
--build flag, which changes the compiler to act more like an
orchestrator than a simple compiler.

Bug: 1002798
Change-Id: I831aaaa9a0f159d6e90b9ec698ba5cc1f7653c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274387Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784318}