Add --watch to _macro_tool for analyzer.

[davidmorgan]

So we get a first idea where we are w.r.t. performance #94.

I'm a little surprised the analyzer integration already works for changes to the script file, since we didn't test that :) as far as I can tell it just sends additional AugmentationRequest instances to the macro, no rebuild/relaunch of the macro.

I am not surprised that it looks fairly slow right now :) 2.7 seconds for a rerun+reanalyze of json_codable_test.dart.

dart ~/git/macros/pkgs/_macro_tool/bin/main.dart --workspace=. --packageConfig=../../.dart_tool/package_config.json --script=lib/json_codable_test.dart --host=analyzer --watch
Running json_codable_test.dart with macros on analyzer.
~~~
Package config: /usr/local/google/home/davidmorgan/git/macros/.dart_tool/package_config.json
Workspace: /usr/local/google/home/davidmorgan/git/macros/goldens/foo
Script: /usr/local/google/home/davidmorgan/git/macros/goldens/foo/lib/json_codable_test.dart
Resolved in 9734ms.
Macro output (patched to use augment library): /usr/local/google/home/davidmorgan/git/macros/goldens/foo/lib/json_codable_test.dart.macro_tool_output
Running with --watch, waiting for next change to script.
Script changed, rerunning macro.
Resolved in 2775ms.
Macro output (patched to use augment library): /usr/local/google/home/davidmorgan/git/macros/goldens/foo/lib/json_codable_test.dart.macro_tool_output
Running with --watch, waiting for next change to script.
Script changed, rerunning macro.
Resolved in 2705ms.