Provide more widget const constructors. (#9247)

Still many more to go.

packages/flutter/lib/src/painting/fractional_offset.dart

@@ -14,7 +14,9 @@ class FractionalOffset {
   /// Creates a fractional offset.
   ///
   /// The [dx] and [dy] arguments must not be null.
-  const FractionalOffset(this.dx, this.dy);
+  const FractionalOffset(this.dx, this.dy)
+    : assert(dx != null),
+      assert(dy != null);
 
   /// The distance fraction in the horizontal direction.
   ///

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

@@ -167,19 +167,18 @@ class Banner extends StatelessWidget {
   /// Creates a banner.
   ///
   /// The [message] and [location] arguments must not be null.
-  Banner({
+  const Banner({
     Key key,
     this.child,
     @required this.message,
     @required this.location,
     this.color: _kColor,
     this.textStyle: _kTextStyle,
-  }) : super(key: key) {
-    assert(message != null);
-    assert(location != null);
-    assert(color != null);
-    assert(textStyle != null);
-  }
+  }) : assert(message != null),
+       assert(location != null),
+       assert(color != null),
+       assert(textStyle != null),
+       super(key: key);
 
   /// The widget to show behind the banner.
   final Widget child;
@@ -215,7 +214,7 @@ class Banner extends StatelessWidget {
 /// Does nothing in release mode.
 class CheckedModeBanner extends StatelessWidget {
   /// Creates a checked mode banner.
-  CheckedModeBanner({
+  const CheckedModeBanner({
     Key key,
     @required this.child
   }) : super(key: key);

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

@@ -1339,9 +1339,9 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
   /// The [children] argument must not be null and must not contain any null
   /// objects.
   MultiChildRenderObjectWidget({ Key key, this.children: const <Widget>[] })
-    : super(key: key) {
-    assert(children != null);
-    assert(!children.any((Widget child) => child == null));
+    : assert(children != null),
+      super(key: key) {
+    assert(!children.any((Widget child) => child == null)); // https://github.com/dart-lang/sdk/issues/29276
   }
 
   /// The widgets below this widget in the tree.