Move assert(s) that reference 'this' to the constructor bodies. (#66914)

packages/flutter/lib/src/cupertino/route.dart

@@ -349,8 +349,9 @@ class CupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTransitionMi
   }) : assert(builder != null),
        assert(maintainState != null),
        assert(fullscreenDialog != null),
-       assert(opaque),
-       super(settings: settings, fullscreenDialog: fullscreenDialog);
+       super(settings: settings, fullscreenDialog: fullscreenDialog) {
+    assert(opaque);
+  }
 
   /// Builds the primary contents of the route.
   final WidgetBuilder builder;
@@ -376,8 +377,9 @@ class _PageBasedCupertinoPageRoute<T> extends PageRoute<T> with CupertinoRouteTr
   _PageBasedCupertinoPageRoute({
     required CupertinoPage<T> page,
   }) : assert(page != null),
-       assert(opaque),
-       super(settings: page);
+       super(settings: page) {
+    assert(opaque);
+  }
 
   CupertinoPage<T> get _page => settings as CupertinoPage<T>;
 

packages/flutter/lib/src/material/page.dart

@@ -45,8 +45,9 @@ class MaterialPageRoute<T> extends PageRoute<T> with MaterialRouteTransitionMixi
   }) : assert(builder != null),
        assert(maintainState != null),
        assert(fullscreenDialog != null),
-       assert(opaque),
-       super(settings: settings, fullscreenDialog: fullscreenDialog);
+       super(settings: settings, fullscreenDialog: fullscreenDialog) {
+    assert(opaque);
+  }
 
   /// Builds the primary contents of the route.
   final WidgetBuilder builder;
@@ -186,8 +187,9 @@ class _PageBasedMaterialPageRoute<T> extends PageRoute<T> with MaterialRouteTran
   _PageBasedMaterialPageRoute({
     @required MaterialPage<T> page,
   }) : assert(page != null),
-       assert(opaque),
-       super(settings: page);
+       super(settings: page) {
+    assert(opaque);
+  }
 
   MaterialPage<T> get _page => settings as MaterialPage<T>;
 

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

@@ -1918,6 +1918,7 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
   /// objects.
   MultiChildRenderObjectWidget({ Key? key, this.children = const <Widget>[] })
     : assert(children != null),
+      super(key: key) {
     assert(() {
       for (int index = 0; index < children.length; index++) {
         // TODO(a14n): remove this check to have a lot more const widget
@@ -1929,8 +1930,8 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
         }
       }
       return true;
-      }()), // https://github.com/dart-lang/sdk/issues/29276
-      super(key: key);
+    }()); // https://github.com/dart-lang/sdk/issues/29276
+  }
 
   /// The widgets below this widget in the tree.
   ///