Use a simpler implementation of Diagnosticable.toString when running in...

Use a simpler implementation of Diagnosticable.toString when running in profile/release mode (#28661)

Diagnosticable.toString may be called in performance sensitive contexts
(for example, creating debug labels for Tickers)

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -2127,7 +2127,12 @@ abstract class Diagnosticable {
 
   @override
   String toString({ DiagnosticLevel minLevel = DiagnosticLevel.debug }) {
-    return toDiagnosticsNode(style: DiagnosticsTreeStyle.singleLine).toString(minLevel: minLevel);
+    String fullString;
+    assert(() {
+      fullString = toDiagnosticsNode(style: DiagnosticsTreeStyle.singleLine).toString(minLevel: minLevel);
+      return true;
+    }());
+    return fullString ?? toStringShort();
   }
 
   /// Returns a debug representation of the object that is used by debugging