1. Remove unused dart debugger API entrypoints which are not used anymore

   as this API has been deprecated and dartium was the last user.

2. Some unit tests are using some of these API entrypoints, so moved them
   over to a test file which will be linked into run_vm_tests

Change-Id: I3343b23d082400da3c803cf81ea80dfaa5e426cf
Reviewed-on: https://dart-review.googlesource.com/7942
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

Author: a-siva

runtime/BUILD.gn

@@ -187,7 +187,6 @@ template("libdart_library") {
       "include/dart_native_api.h",
       "include/dart_tools_api.h",
       "vm/dart_api_impl.cc",
-      "vm/debugger_api_impl.cc",
       "vm/mirrors_api_impl.cc",
       "vm/native_api_impl.cc",
       "vm/version.h",