[test] Fixing hot reload suite diff checking.

Should resolve flaky cross-platform diffs

Change-Id: Ic496829ea984d049f8fe984e614aaf4fb41c283c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/402941
Commit-Queue: Mark Zhou <[email protected]>
Auto-Submit: Mark Zhou <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: Markzipan

pkg/dev_compiler/test/hot_reload_suite.dart

@@ -930,8 +930,10 @@ abstract class HotReloadSuiteRunner {
   /// 'diff' can be unstable across platforms around newline offsets.
   (String, String) _filterLineDeltas(String diff1, String diff2) {
     bool isBlankLineOrDelta(String s) {
-      return s.trim().isEmpty ||
-          (s.startsWith('+') || s.startsWith('-')) && s.trim().length == 1;
+      var trimmed = s.trim();
+      return trimmed.isEmpty ||
+          (trimmed.startsWith('+') || trimmed.startsWith('-')) &&
+              trimmed.length == 1;
     }
 
     var diff1Lines = LineSplitter().convert(diff1)