Modify stepping integration test to accommodate new DDC async semantics. (flutter#152204)

Updates the expected steps in the async function defined within `stepping_project.dart`. 

The Dart web team is updating the async semantics of DDC to bring them in line with the other backends. Currently, the DDC async semantics don't adhere to the Dart spec and this can lead to inconsistent and surprising results.

However, the step-over operation doesn't work well yet with the new DDC async semantics. In the long run we intend to improve this but until then the debug stepper will have sporadic results that we can't model well with this test. When we are able to fix the stepper functionality, we will return this test to cover more of the async function being stepped over.

Author: biggs0125

packages/flutter_tools/test/integration.shard/test_data/stepping_project.dart

@@ -94,10 +94,10 @@ class WebSteppingProject extends Project {
     Future<void> doAsyncStuff() async {
       print("test"); // BREAKPOINT
       await Future.value(true); // STEP 1
-      await Future.microtask(() => true); // STEP 2
-      await Future.delayed(const Duration(milliseconds: 1));  // STEP 3
-      print("done!"); // STEP 4
-    } // STEP 5
+      await Future.microtask(() => true);
+      await Future.delayed(const Duration(milliseconds: 1));
+      print("done!");
+    }
 
     @override
     Widget build(BuildContext context) {
@@ -113,5 +113,5 @@ class WebSteppingProject extends Project {
   int get breakpointLine => lineContaining(main, '// BREAKPOINT');
   int lineForStep(int i) => lineContaining(main, '// STEP $i');
 
-  final int numberOfSteps = 5;
+  final int numberOfSteps = 1;
 }