[VM/runtime] Remove support for loading URLs through service isolate

- Removes the load port functionality from the service isolate, this was
  used to load sources in Dart 1 and is pretty much dead code in Dart 2
- Moves resolution of URIs and Package URI (based on package map) to the
  individual isolates instead of sending a request to the service isolate
- Setups the package map lazily as it is needed only if resolveURI is
  called in Dart code, it is not needed for loading/compiling sources
  (this is now done in the front end).
- Removed Dart_ServiceWaitForLoadPort() methods from the Dart C API as
  it is not used anywhere.

Change-Id: I6c3704bc431bdcd49bf074eb58bee1ed492ccccb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/129742
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

PRESUBMIT.py

@@ -247,7 +247,7 @@ def _CheckClangTidy(input_api, output_api):
     files = []
     for f in input_api.AffectedFiles():
         path = f.LocalPath()
-        if is_cpp_file(path): files.append(path)
+        if is_cpp_file(path) and os.path.isfile(path): files.append(path)
 
     if not files:
         return []