Add implicit animations to Input

Now the label animates from its inline position to above the text and
the focus highlight grows into place.

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
@@ -73,6 +74,9 @@ class Input extends StatefulComponent {
   _InputState createState() => new _InputState();
 }
 
+const Duration _kTransitionDuration = const Duration(milliseconds: 200);
+const Curve _kTransitionCurve = Curves.ease;
+
 class _InputState extends State<Input> {
   String _value;
   EditableString _editableString;
@@ -139,21 +143,34 @@ class _InputState extends State<Input> {
 
     List<Widget> stackChildren = <Widget>[];
 
+    bool hasInlineLabel = config.labelText != null && !focused && !_value.isNotEmpty;
+
     if (config.labelText != null) {
-      TextStyle labelStyle = themeData.text.caption.copyWith(color: focused ? focusHighlightColor : themeData.hintColor);
-      stackChildren.add(new Positioned(
+      TextStyle labelStyle = hasInlineLabel ?
+        themeData.text.subhead.copyWith(color: themeData.hintColor) :
+        themeData.text.caption.copyWith(color: focused ? focusHighlightColor : themeData.hintColor);
+
+      double topPaddingIncrement = themeData.text.caption.fontSize + (config.isDense ? 4.0 : 8.0);
+      double top = topPadding;
+      if (hasInlineLabel)
+        top += topPaddingIncrement + textStyle.fontSize - labelStyle.fontSize;
+
+      stackChildren.add(new AnimatedPositioned(
         left: 0.0,
-        top: topPadding,
+        top: top,
+        duration: _kTransitionDuration,
+        curve: _kTransitionCurve,
         child: new Text(config.labelText, style: labelStyle)
       ));
-      topPadding += labelStyle.fontSize + (config.isDense ? 4.0 : 8.0);
+
+      topPadding += topPaddingIncrement;
     }
 
-    if (config.hintText != null && _value.isEmpty) {
-      TextStyle hintStyle = textStyle.copyWith(color: themeData.hintColor);
+    if (config.hintText != null && _value.isEmpty && !hasInlineLabel) {
+      TextStyle hintStyle = themeData.text.subhead.copyWith(color: themeData.hintColor);
       stackChildren.add(new Positioned(
         left: 0.0,
-        top: topPadding,
+        top: topPadding + textStyle.fontSize - hintStyle.fontSize,
         child: new Text(config.hintText, style: hintStyle)
       ));
     }
@@ -176,9 +193,11 @@ class _InputState extends State<Input> {
       }
     }
 
-    stackChildren.add(new Container(
+    stackChildren.add(new AnimatedContainer(
       margin: margin,
       padding: padding,
+      duration: _kTransitionDuration,
+      curve: _kTransitionCurve,
       decoration: new BoxDecoration(
         border: new Border(
           bottom: new BorderSide(

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

@@ -29,6 +29,25 @@ class BorderSide {
   /// A black border side of zero width.
   static const none = const BorderSide(width: 0.0);
 
+  BorderSide copyWith({
+    Color color,
+    double width
+  }) {
+    return new BorderSide(
+      color: color ?? this.color,
+      width: width ?? this.width
+    );
+  }
+
+  static BorderSide lerp(BorderSide a, BorderSide b, double t) {
+    assert(a != null);
+    assert(b != null);
+    return new BorderSide(
+      color: Color.lerp(a.color, b.color, t),
+      width: ui.lerpDouble(a.width, b.width, t)
+    );
+  }
+
   bool operator ==(dynamic other) {
     if (identical(this, other))
       return true;
@@ -79,6 +98,30 @@ class Border {
     return new EdgeDims.TRBL(top.width, right.width, bottom.width, left.width);
   }
 
+  Border scale(double t) {
+    return new Border(
+      top: top.copyWith(width: t * top.width),
+      right: right.copyWith(width: t * right.width),
+      bottom: bottom.copyWith(width: t * bottom.width),
+      left: left.copyWith(width: t * left.width)
+    );
+  }
+
+  static Border lerp(Border a, Border b, double t) {
+    if (a == null && b == null)
+      return null;
+    if (a == null)
+      return b.scale(t);
+    if (b == null)
+      return a.scale(1.0 - t);
+    return new Border(
+      top: BorderSide.lerp(a.top, b.top, t),
+      right: BorderSide.lerp(a.right, b.right, t),
+      bottom: BorderSide.lerp(a.bottom, b.bottom, t),
+      left: BorderSide.lerp(a.left, b.left, t)
+    );
+  }
+
   bool operator ==(dynamic other) {
     if (identical(this, other))
       return true;
@@ -718,7 +761,7 @@ class BoxDecoration extends Decoration {
     return new BoxDecoration(
       backgroundColor: Color.lerp(null, backgroundColor, factor),
       backgroundImage: backgroundImage,
-      border: border,
+      border: Border.lerp(null, border, factor),
       borderRadius: ui.lerpDouble(null, borderRadius, factor),
       boxShadow: BoxShadow.lerpList(null, boxShadow, factor),
       gradient: gradient,
@@ -740,7 +783,7 @@ class BoxDecoration extends Decoration {
     return new BoxDecoration(
       backgroundColor: Color.lerp(a.backgroundColor, b.backgroundColor, t),
       backgroundImage: b.backgroundImage,
-      border: b.border,
+      border: Border.lerp(a.border, b.border, t),
       borderRadius: ui.lerpDouble(a.borderRadius, b.borderRadius, t),
       boxShadow: BoxShadow.lerpList(a.boxShadow, b.boxShadow, t),
       gradient: b.gradient,