Add a test for icon opacity crash

Fixes #2361

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

@@ -63,7 +63,7 @@ class Icon extends StatelessComponent {
     final double iconOpacity = IconTheme.of(context)?.clampedOpacity ?? 1.0;
     Color iconColor = color ?? _getDefaultColor(context);
     if (iconOpacity != 1.0)
-      iconColor = iconColor.withAlpha((255.0 * iconColor.opacity * iconOpacity).round());
+      iconColor = iconColor.withOpacity(iconColor.opacity * iconOpacity);
 
     return new ExcludeSemantics(
       child: new SizedBox(

packages/flutter/test/widget/icon_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Can set opacity for an Icon', () {
+    testWidgets((WidgetTester tester) {
+      tester.pumpWidget(
+        new IconTheme(
+          data: new IconThemeData(
+            color: Colors.green[500],
+            opacity: 0.5
+          ),
+          child: new Icon(icon: Icons.add)
+        )
+      );
+      Text text = tester.findWidgetOfType(Text);
+      expect(text.style.color, equals(Colors.green[500].withOpacity(0.5)));
+    });
+  });
+}

packages/flutter_test/lib/src/instrumentation.dart

@@ -75,6 +75,15 @@ class Instrumentation {
     });
   }
 
+  /// Returns the first [Widget] of the given [runtimeType], if any. Returns
+  /// null if there is no matching widget.
+  Widget findWidgetOfType(Type type) {
+    Element element = findElement((Element element) {
+      return element.widget.runtimeType == type;
+    });
+    return element?.widget;
+  }
+
   /// Returns the [State] object of the first element whose state has
   /// the given [runtimeType], if any. Returns null if there is no
   /// matching element.