Make extended FAB's icon optional (#27711)

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

@@ -92,10 +92,10 @@ class FloatingActionButton extends StatelessWidget {
        disabledElevation = disabledElevation ?? elevation,
        super(key: key);
 
-  /// Creates a wider [StadiumBorder]-shaped floating action button with both
-  /// an [icon] and a [label].
+  /// Creates a wider [StadiumBorder]-shaped floating action button with
+  /// an optional [icon] and a [label].
   ///
-  /// The [label], [icon], [elevation], [highlightElevation], [clipBehavior] and
+  /// The [label], [elevation], [highlightElevation], [clipBehavior] and
   /// [shape] arguments must not be null. Additionally, [elevation]
   /// [highlightElevation], and [disabledElevation] (if specified) must be
   /// non-negative.
@@ -113,7 +113,7 @@ class FloatingActionButton extends StatelessWidget {
     this.isExtended = true,
     this.materialTapTargetSize,
     this.clipBehavior = Clip.none,
-    @required Widget icon,
+    Widget icon,
     @required Widget label,
   }) : assert(elevation != null && elevation >= 0.0),
        assert(highlightElevation != null && highlightElevation >= 0.0),
@@ -127,13 +127,19 @@ class FloatingActionButton extends StatelessWidget {
        child = _ChildOverflowBox(
          child: Row(
            mainAxisSize: MainAxisSize.min,
-           children: <Widget>[
-             const SizedBox(width: 16.0),
-             icon,
-             const SizedBox(width: 8.0),
-             label,
-             const SizedBox(width: 20.0),
-           ],
+           children: icon == null
+             ? <Widget>[
+                 const SizedBox(width: 20.0),
+                 label,
+                 const SizedBox(width: 20.0),
+               ]
+             : <Widget>[
+                 const SizedBox(width: 16.0),
+                 icon,
+                 const SizedBox(width: 8.0),
+                 label,
+                 const SizedBox(width: 20.0),
+               ],
          ),
        ),
        super(key: key);

packages/flutter/test/material/floating_action_button_test.dart

@@ -338,6 +338,49 @@ void main() {
     expect(tester.getSize(fabFinder).width, 168);
   });
 
+  testWidgets('FloatingActionButton.isExtended (without icon)', (WidgetTester tester) async {
+    final Finder fabFinder = find.byType(FloatingActionButton);
+
+    FloatingActionButton getFabWidget() {
+      return tester.widget<FloatingActionButton>(fabFinder);
+    }
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton.extended(
+            label: const SizedBox(
+              width: 100.0,
+              child: Text('label'),
+            ),
+            onPressed: null,
+          ),
+        ),
+      ),
+    );
+
+    expect(getFabWidget().isExtended, true);
+    expect(getFabWidget().shape, const StadiumBorder());
+    expect(find.text('label'), findsOneWidget);
+    expect(find.byType(Icon), findsNothing);
+
+    // Verify that the widget's height is 48 and that its internal
+    /// horizontal layout is: 20 label 20
+    expect(tester.getSize(fabFinder).height, 48.0);
+
+    final double fabLeft = tester.getTopLeft(fabFinder).dx;
+    final double fabRight = tester.getTopRight(fabFinder).dx;
+    final double labelLeft = tester.getTopLeft(find.text('label')).dx;
+    final double labelRight = tester.getTopRight(find.text('label')).dx;
+    expect(labelLeft - fabLeft, 20.0);
+    expect(fabRight - labelRight, 20.0);
+
+    // The overall width of the button is:
+    // 140 = 20 + 100(label) + 20
+    expect(tester.getSize(find.text('label')).width, 100.0);
+    expect(tester.getSize(fabFinder).width, 140);
+  });
+
   testWidgets('Floating Action Button heroTag', (WidgetTester tester) async {
     BuildContext theContext;
     await tester.pumpWidget(