Stop passing a nullable value to Future<nn-type>.value or Completer<n…

…n-type>.completer (#6228)

* Stop passing a nullable value to Future<nn-type>.value or Completer<nn-type>.completer.

This is cleanup work required to start enforcing this with static analysis, as per dart-lang/sdk#53253.

Real quick this issue is that this code is unsafe:

```dart
void f(Completer<int> c, int? i) {
  Future<int>.value(i); // Ouch!
  c.complete(i);        // Ouch!
}
```

The typical fix is to add a null-assert (`!`), but sometimes a more appropriate or safer fix can be made.

* Use non-nullable CpuSamples

packages/devtools_app/lib/src/shared/scripts/script_manager.dart

@@ -105,8 +105,9 @@ class _ScriptCache {
     ScriptRef scriptRef,
   ) {
     final scriptId = scriptRef.id!;
-    if (_scripts.containsKey(scriptId)) {
-      return Future.value(_scripts[scriptId]);
+    final script = _scripts[scriptId];
+    if (script != null) {
+      return Future.value(script);
     }
 
     if (_inProgress.containsKey(scriptId)) {

packages/devtools_test/lib/src/mocks/fake_service_manager.dart

@@ -161,7 +161,7 @@ class FakeServiceManager extends Fake implements ServiceConnectionManager {
         Response.parse({
           'layerBytes': 0,
           'pictureBytes': 0,
-        }),
+        })!,
       );
 
   @override
@@ -192,7 +192,7 @@ class FakeServiceManager extends Fake implements ServiceConnectionManager {
         'dartSdkVersion': '2.9.0 (build 2.9.0-8.0.dev d6fed1f624)',
         'frameworkRevisionShort': '74432fa91c',
         'engineRevisionShort': 'ae2222f47e',
-      }),
+      })!,
     );
   }
 

packages/devtools_test/lib/src/mocks/fake_vm_service_wrapper.dart

@@ -46,9 +46,9 @@ class FakeVmServiceWrapper extends Fake implements VmServiceWrapper {
     'pid': 54321,
     'functions': [],
     'samples': [],
-  });
+  })!;
 
-  CpuSamples? cpuSamples;
+  CpuSamples cpuSamples;
 
   CpuSamples? allocationSamples;
 
@@ -249,7 +249,7 @@ class FakeVmServiceWrapper extends Fake implements VmServiceWrapper {
             },
           ),
         },
-      ),
+      )!,
     );
   }
 
@@ -338,7 +338,7 @@ class FakeVmServiceWrapper extends Fake implements VmServiceWrapper {
 
   @override
   Future<TimelineFlags> getVMTimelineFlags() =>
-      Future.value(TimelineFlags.parse(_vmTimelineFlags));
+      Future.value(TimelineFlags.parse(_vmTimelineFlags)!);
 
   @override
   Future<Timeline> getVMTimeline({
@@ -403,7 +403,7 @@ class FakeVmServiceWrapper extends Fake implements VmServiceWrapper {
   ) async {
     final httpProfile = await getHttpProfile(isolateId);
     return Future.value(
-      httpProfile.requests.firstWhereOrNull((request) => request.id == id),
+      httpProfile.requests.firstWhere((request) => request.id == id),
     );
   }