[vm] Heap snapshot tool can load file from arguments

This CL allows the heap snapshot tool to load a file as it is launched
via a command like
```
../../../out/ReleaseX64/dart bin/explore.dart my_snapshot.heapsnapshot
```

Change-Id: I4eca4ec18daef4d3361c754018d234045e725ef5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/275860
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

runtime/tools/heapsnapshot/bin/explore.dart

@@ -50,7 +50,7 @@ class CompletionKeyHandler extends KeyHandler {
   }
 }
 
-void main() async {
+Future<void> main(List<String> args) async {
   final console = SmartConsole();
 
   console.write('The ');
@@ -67,6 +67,24 @@ void main() async {
 
   console.completionHandler = CompletionKeyHandler(cliState);
 
+  if (args.isNotEmpty) {
+    if (args.length == 1 && args.single.trim().isNotEmpty) {
+      console.setForegroundColor(ConsoleColor.brightYellow);
+      console.writeLine('Will try to load ${args.single.trim()}.');
+      console.resetColorAttributes();
+      console.writeLine('');
+      if (await cliCommandRunner.run(cliState, ['load', args.single.trim()])) {
+        return;
+      }
+    } else {
+      console.setForegroundColor(ConsoleColor.brightRed);
+      console.writeLine('When giving arguments, only one argument - '
+          'the snapshot to load - is supported. Ignoring arguments.');
+      console.resetColorAttributes();
+      console.writeLine('');
+    }
+  }
+
   while (true) {
     final response = console.smartReadLine();
     console.resetColorAttributes();