title: unable to add gradient to TextFormField in flutter
body: <p>i am trying to add gradient color to my TextFormField but unable to do since i am not sure how to do it.</p>
  
  <p>below is my code so far</p>
  
  <pre><code>
  
      return Scaffold(
        appBar: AppBar(
          title: Text('Simple Interest Calculator'),
        ),
        body: Form(
          key: _formKey,
          child: Column (
            children: [
              Padding(
                padding: EdgeInsets.only(top: 10.0, bottom: 5.0, left: 15.0, right: 15.0),
                child: TextFormField(
          keyboardType: textInputType,
          style: Theme
                .of(context)
                .textTheme
                .title,
          controller: textController,
           decoration: InputDecoration(
             labelStyle: TextStyle(
                    color: Colors.grey,
                    fontSize: 16.0
             ),
             contentPadding: EdgeInsets.fromLTRB(10.0, 10.0, 10.0, 10.0),
               errorStyle: TextStyle(
               color: Colors.red,
               fontSize: 15.0
             ),
             enabledBorder: OutlineInputBorder(
               borderSide:  BorderSide(color: colorTextInput, width:1.0),
               borderRadius: BorderRadius.circular(5.0)
             )
           )
      )
              ),
  
  
        ),
      );
    }
  
  </code></pre>
  
  <p>i am looking to have a gradient similar to the picture below.  instead of gradient going from top to bottom or bottom to top.  i am looking for gradient left to right.  i want to start with color "#fafafa" in left side and end with color "#EDEDED" in the right side of the textfield.  can someone help me on how to do that in flutter? thanks in advance</p>
  
  <p><a href="https://i.stack.imgur.com/Ozglk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ozglk.png" alt="enter image description here"></a></p>