[ddc] Test DartPad-like hot reload workflow.

Refactors the hot reload test suite to support non-FE server based hot reload. Similar to DartPad it uses a DDC process running in '--persistent_worker' mode and sends bazel requests for each reload.

I call this "stateless" mode because the compilation process itself is not maintaining any state. The necessary metadata is passed from one compilation to the other via a delta dill. This differs from the "stateful" mode where the FE server persists a kernel AST in memory from one compilation to the next.

Disregarding the browsers there are effectively 3 run modes now:
- web stateful
- web stateless (new)
- vm stateful

One key difference between the "stateless" and "stateful" modes is the output format of the JS files. In stateful mode DDC emits a file per library being re-compiled. In stateless mode DDC is emitting a single file with all the libraries.

DartPad's workflow is slightly simpler than what's seen in the stateless mode here. It only supports editing a single library so we skip processing any metadata. To simulate this I've added the special 'main_only' which passes a single library for each reload generation. I've verified that this would fail if not for the change recently made to ddc_module_loader.js.

Change-Id: If05da6dbeded4dd20e9e6d9dfaed52151541a19b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434340
Commit-Queue: Nate Biggs <[email protected]>
Reviewed-by: Mark Zhou <[email protected]>
Reviewed-by: Kevin Moore <[email protected]>
Reviewed-by: Nicholas Shahan <[email protected]>

Author: natebiggs

pkg/dev_compiler/lib/src/command/command.dart

@@ -394,8 +394,8 @@ Future<CompilerResult> _compile(
   } else {
     if (reloadLastAcceptedKernel != null) {
       throw ArgumentError(
-        "Must provide 'new-reload-delta-kernel' if "
-        "'old-reload-delta-kernel' provided.",
+        "Must provide 'reload-delta-kernel' if "
+        "'reload-last-accepted-kernel' provided.",
       );
     }
   }