next-coder
diff --git a/‎packages/flutter/lib/material.dart
+1 b/‎packages/flutter/lib/material.dart
+1
diff --git a/‎packages/flutter/lib/src/material/tab_indicator.dart
+95 b/‎packages/flutter/lib/src/material/tab_indicator.dart
+95
@@ -81,6 +81,7 @@ export 'src/material/stepper.dart';
 export 'src/material/switch.dart';
 export 'src/material/switch_list_tile.dart';
 export 'src/material/tab_controller.dart';
+export 'src/material/tab_indicator.dart';
 export 'src/material/tabs.dart';
 export 'src/material/text_field.dart';
 export 'src/material/text_form_field.dart';
 
@@ -0,0 +1,95 @@
+// Copyright 2018 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/widgets.dart';
+
+import 'colors.dart';
+
+/// Used with [TabBar.indicator] to draw a horizontal line below the
+/// selected tab.
+///
+/// The selected tab underline is inset from the tab's boundary by [insets].
+/// The [borderSide] defines the line's color and weight.
+///
+/// The [TabBar.indicatorSize] property can be used to define the indicator's
+/// bounds in terms of its (centered) widget with [TabIndicatorSize.label],
+/// or the entire tab with [TabIndicatorSize.tab].
+class UnderlineTabIndicator extends Decoration {
+  /// Create an underline style selected tab indicator.
+  ///
+  /// The [borderSide] and [insets] arguments must not be null.
+  const UnderlineTabIndicator({
+    this.borderSide: const BorderSide(width: 2.0, color: Colors.white),
+    this.insets: EdgeInsets.zero,
+  }) : assert(borderSide != null), assert(insets != null);
+
+  /// The color and weight of the horizontal line drawn below the selected tab.
+  final BorderSide borderSide;
+
+  /// Locates the selected tab's underline relative to the tab's boundary.
+  ///
+  /// The [TabBar.indicatorSize] property can be used to define the
+  /// tab indicator's bounds in terms of its (centered) tab widget with
+  /// [TabIndicatorSize.label], or the entire tab with [TabIndicatorSize.tab].
+  final EdgeInsetsGeometry insets;
+
+  @override
+  Decoration lerpFrom(Decoration a, double t) {
+    if (a is UnderlineTabIndicator) {
+      return new UnderlineTabIndicator(
+        borderSide: BorderSide.lerp(a.borderSide, borderSide, t),
+        insets: EdgeInsetsGeometry.lerp(a.insets, insets, t),
+      );
+    }
+    return super.lerpFrom(a, t);
+  }
+
+  @override
+  Decoration lerpTo(Decoration b, double t) {
+    if (b is UnderlineTabIndicator) {
+      return new UnderlineTabIndicator(
+        borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
+        insets: EdgeInsetsGeometry.lerp(insets, b.insets, t),
+      );
+    }
+    return super.lerpTo(b, t);
+  }
+
+  @override
+  _UnderlinePainter createBoxPainter([VoidCallback onChanged]) {
+    return new _UnderlinePainter(this, onChanged);
+  }
+}
+
+class _UnderlinePainter extends BoxPainter {
+  _UnderlinePainter(this.decoration, VoidCallback onChanged)
+    : assert(decoration != null), super(onChanged);
+
+  final UnderlineTabIndicator decoration;
+
+  BorderSide get borderSide => decoration.borderSide;
+  EdgeInsetsGeometry get insets => decoration.insets;
+
+  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
+    assert(rect != null);
+    assert(textDirection != null);
+    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);
+    return new Rect.fromLTWH(
+      indicator.left,
+      indicator.bottom - borderSide.width,
+      indicator.width,
+      borderSide.width,
+    );
+  }
+
+  @override
+  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
+    assert(configuration != null);
+    assert(configuration.size != null);
+    final Rect rect = offset & configuration.size;
+    final TextDirection textDirection = configuration.textDirection;
+    final Rect indicator = _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
+    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, borderSide.toPaint());
+  }
+}