made top level if checks gaurd clauses (flutter#135070)

This is a tiny tweak to replace some top level if clauses with guard clauses in `FutureBuilder`. I find the resultant code much more readable, but this is a matter of taste and I didn't see any info one way or another on it in the style guide so let me know if this is not to your all's preference.

packages/flutter/lib/src/widgets/async.dart

@@ -595,13 +595,14 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   @override
   void didUpdateWidget(FutureBuilder<T> oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (oldWidget.future != widget.future) {
-      if (_activeCallbackIdentity != null) {
-        _unsubscribe();
-        _snapshot = _snapshot.inState(ConnectionState.none);
-      }
-      _subscribe();
+    if (oldWidget.future == widget.future) {
+      return;
+    }
+    if (_activeCallbackIdentity != null) {
+      _unsubscribe();
+      _snapshot = _snapshot.inState(ConnectionState.none);
     }
+    _subscribe();
   }
 
   @override
@@ -614,33 +615,35 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   }
 
   void _subscribe() {
-    if (widget.future != null) {
-      final Object callbackIdentity = Object();
-      _activeCallbackIdentity = callbackIdentity;
-      widget.future!.then<void>((T data) {
-        if (_activeCallbackIdentity == callbackIdentity) {
-          setState(() {
-            _snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);
-          });
-        }
-      }, onError: (Object error, StackTrace stackTrace) {
-        if (_activeCallbackIdentity == callbackIdentity) {
-          setState(() {
-            _snapshot = AsyncSnapshot<T>.withError(ConnectionState.done, error, stackTrace);
-          });
-        }
-        assert(() {
-          if (FutureBuilder.debugRethrowError) {
-            Future<Object>.error(error, stackTrace);
-          }
-          return true;
-        }());
-      });
-      // An implementation like `SynchronousFuture` may have already called the
-      // .then closure. Do not overwrite it in that case.
-      if (_snapshot.connectionState != ConnectionState.done) {
-        _snapshot = _snapshot.inState(ConnectionState.waiting);
+    if (widget.future == null) {
+      // There is no future to subscribe to, do nothing.
+      return;
+    }
+    final Object callbackIdentity = Object();
+    _activeCallbackIdentity = callbackIdentity;
+    widget.future!.then<void>((T data) {
+      if (_activeCallbackIdentity == callbackIdentity) {
+        setState(() {
+          _snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);
+        });
       }
+    }, onError: (Object error, StackTrace stackTrace) {
+      if (_activeCallbackIdentity == callbackIdentity) {
+        setState(() {
+          _snapshot = AsyncSnapshot<T>.withError(ConnectionState.done, error, stackTrace);
+        });
+      }
+      assert(() {
+        if (FutureBuilder.debugRethrowError) {
+          Future<Object>.error(error, stackTrace);
+        }
+        return true;
+      }());
+    });
+    // An implementation like `SynchronousFuture` may have already called the
+    // .then closure. Do not overwrite it in that case.
+    if (_snapshot.connectionState != ConnectionState.done) {
+      _snapshot = _snapshot.inState(ConnectionState.waiting);
     }
   }