[analysis_server] Use sdk.dart instead of Platform.resolvedExecutable

Sam noted that AOT snapshots might return `dartaotruntime` for `Platform.resolvedExecutable`. It doesn't seem to happen if you invoke server with `dart language_server` because `dart` is still the executable, but if you invoked the snapshot directly with `dartaotruntime` then this would fail.

To avoid any possibly issues, this changes it to use the same `sdk.dart` getter that some other code uses that handles this difference by constructed the path to the `dart` executable.

Change-Id: I099335a255a792b6754d8d2a1f0bde491c09699e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/464681
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/services/pub/pub_command.dart

@@ -7,6 +7,7 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:analysis_server/src/utilities/process.dart';
+import 'package:analysis_server/src/utilities/sdk.dart';
 import 'package:analyzer/instrumentation/service.dart';
 import 'package:path/path.dart' as path;
 
@@ -120,7 +121,7 @@ class PubCommand {
     try {
       _instrumentationService.logInfo('Starting pub command $args');
       var process = await _processRunner.start(
-        Platform.resolvedExecutable,
+        sdk.dart,
         ['pub', ...args],
         workingDirectory: workingDirectory,
         environment: {_pubEnvironmentKey: _pubEnvironmentValue},