Pass filename instead of piping contents through stdin to dart format in

PRESUBMIT.py

FIXES=#46600

Change-Id: I69358185b5a8dfc863f17cd4beb0209592d3be09
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206681
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

PRESUBMIT.py

@@ -133,17 +133,13 @@ def _CheckDartFormat(input_api, output_api):
             dart,
             'format',
             '--set-exit-if-changed',
+            '--output=none',
+            '--summary=none',
+            filename,
         ]
-        if not contents:
-            args += [
-                '--output=none',
-                '--summary=none',
-                filename,
-            ]
 
         process = subprocess.Popen(
             args, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
-        process.communicate(input=contents)
 
         # Check for exit code 1 explicitly to distinguish it from a syntax error
         # in the file (exit code 65). The repo contains many Dart files that are