Allow asset transformers to register additional input dependencies for hot reload by passing a `--depfile` flag to the transformer subprocess and parsing the resulting Ninja-format depfile.

Key changes:
- `AssetTransformer` now passes `--depfile` and parses it using `DepfileService`, returning dependencies in a new `AssetTransformationResult`.
- `DevelopmentAssetTransformer` tracks these dependencies and provides them to `DevFS`.
- `DevFS.updateBundle` does not skip transformed assets on startup, running the transformer to collect dependencies but optimizing by not uploading them to the device unless requested (e.g., for Web).
- `DevFS.updateBundle` checks if any tracked dependencies are invalidated and triggers re-transformation.
- `DevFS.update` and `WebDevFS.update` append collected dependencies to the watched `sources` set.
- `copyAssets` build target collects and adds these dependencies to its inputs to ensure correct rebuilds.
- Added unit and integration tests to verify the behavior.

Fixes flutter#187051

Update existing tests in `assets_test.dart` to expect the `--depfile` argument in `FakeCommand`s and handle it in `ArgParser`s.

Also fix the `Uses processors~/2 to transform assets` test to expect 5 transformed assets instead of 4, which is the correct number of assets (1 primary + 4 variants) created and processed in the test.

Omit obvious local variable types for `processManager` and `environment` in `Uses processors~/2 to transform assets` test.

Update `FakeCommand` and `onRun` in `bundle_builder_test.dart` to support the new `--depfile` argument.

…transformers

Redesign `AssetTransformer` to pass the depfile path via `FLUTTER_ASSET_TRANSFORMER_DEPFILE` environment variable instead of `--depfile` command-line argument. This ensures 100% backwards-compatibility with existing or user-defined transformers that do not support the `--depfile` argument and would otherwise crash.

Make `invalidatedFiles` optional in `DevFS.updateBundle` to maintain compatibility with existing tests and callers that do not pass it.

Update `asset_transformer_test.dart` to verify the new environment variable behavior, using a robust temp directory filtering approach in the depfile parsing test.

Fix a latent bug in `assets_test.dart` where the `Uses processors~/2` test expected 4 transformed assets instead of 5 (1 primary + 4 variants).

Revert other test files (`devfs_test.dart`, `bundle_builder_test.dart`, and other tests in `assets_test.dart`) to their `origin/main` state since they no longer need to mock the `--depfile` argument.

Update `DepfileService.parse` to accept an optional `baseDirectory` parameter, which is used to resolve relative paths found in the depfile.

Pass the `workingDirectory` (project root) as `baseDirectory` to `depfileService.parse` in `AssetTransformer` to ensure relative paths written by transformers are resolved correctly.

Add a unit test to `depfile_test.dart` to verify relative path resolution against a base directory.

Rewrite `AssetTransformer` to use an implicit naming convention for depfiles instead of an environment variable.

Now, if a transformer wants to register dependencies, it should write its depfile to the output path with `.d` appended (i.e., `<output>.d`). `AssetTransformer` checks for the existence of this file after running the transformer, parses it, and registers the dependencies.

This approach:
- Is 100% backwards-compatible with existing transformers (no new arguments or env vars are passed).
- Eliminates the need for environment variables.
- Simplifies the code by removing the second temporary directory.

Update `asset_transformer_test.dart` to match this new convention.

In the `Uses processors~/2 to transform assets` test, assert the length of `inputFilePaths` and `outputFilePaths` lists directly instead of calling `toSet()`.

`MemoryFileSystem` can reuse deleted temporary directory names sequentially. Since the test runs multiple transformations, some directories are deleted before others start, leading to name reuse (e.g., two runs using `/.tmp_rand0/rand1/`). Calling `toSet()` deduplicated these paths and caused the assertion to fail with a length of 4 instead of 5 in some environments (like CI).

Asserting the list length directly is more robust and correctly verifies that all 5 transformations were executed.