[dartdevc] clear console on hot restart

Internal request b/147818704

Change-Id: I08c655c74ea10b6f14ffde8283cbfc286dfae41b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132180
Commit-Queue: Vijay Menon <vsm@google.com>
Reviewed-by: Gary Roumanis <grouma@google.com>

pkg/dev_compiler/lib/js/legacy/dart_library.js

@@ -246,6 +246,7 @@ if (!dart_library) {
 
     /// Once the `onReloadStart()` completes, this finishes the restart.
     function finishHotRestart() {
+      window.console.clear();
       if (clearState) {
         // This resets all initialized fields and clears type caches and other
         // temporary data structures used by the compiler/SDK.