[dart2js] delete dummy_compiler_test.

This test has been failing for years. The test was meant to
ensure the compiler could run in the browser, but that's not
a priority for the team at the moment.

Fixes #47257

Change-Id: I89b7ddfb51a3a7375526eb4f321f5d9f31a6a405
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/219124
Reviewed-by: Joshua Litt <[email protected]>
Commit-Queue: Sigmund Cherem <[email protected]>

Author: sigmundch