Improve setState-after-dispose error message

We now explicitly mention the possibility of a memory leak.

Fixes #2978

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

@@ -59,8 +59,8 @@ class TextStyle {
   /// The height of this text span, as a multiple of the font size.
   ///
   /// If applied to the root [TextSpan], this value sets the line height, which
-  /// is the minimum distance between each text baselines, as multiple of the
-  /// font size.
+  /// is the minimum distance between subsequent text baselines, as multiple of
+  /// the font size.
   final double height;
 
   /// The decorations to paint near the text (e.g., an underline).

packages/flutter/lib/src/widgets/framework.dart

@@ -394,13 +394,19 @@ abstract class State<T extends StatefulWidget> {
       if (_debugLifecycleState == _StateLifecycle.defunct) {
         throw new FlutterError(
           'setState() called after dispose(): $this\n'
-          'This error happens if you call setState() on State object for a widget that\n'
-          'no longer appears in the widget tree (e.g., whose parent widget no longer\n'
-          'includes the widget in its build). This error can occur when code calls\n'
-          'setState() from a timer or an animation callback. The preferred solution is\n'
-          'to cancel the timer or stop listening to the animation in the dispose()\n'
-          'callback. Another solution is to check the "mounted" property of this\n'
-          'object before calling setState() to ensure the object is still in the tree.'
+          'This error happens if you call setState() on State object for a widget that '
+          'no longer appears in the widget tree (e.g., whose parent widget no longer '
+          'includes the widget in its build). This error can occur when code calls '
+          'setState() from a timer or an animation callback. The preferred solution is '
+          'to cancel the timer or stop listening to the animation in the dispose() '
+          'callback. Another solution is to check the "mounted" property of this '
+          'object before calling setState() to ensure the object is still in the '
+          'tree.\n'
+          '\n'
+          'This error might indicate a memory leak if setState() is being called '
+          'because another object is retaining a reference to this State object '
+          'after it has been removed from the tree. To avoid memory leaks, '
+          'consider breaking the reference to this object during dipose().'
         );
       }
       return true;