api docs: Add descriptions for Scafold, Theme, and ThemeData classes.

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

@@ -175,6 +175,11 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
   }
 }
 
+/// Implements the basic material design visual layout structure.
+///
+/// This class provides APIs for showing drawers, snackbars, and bottom sheets.
+///
+/// See: <https://www.google.com/design/spec/layout/structure.html>
 class Scaffold extends StatefulComponent {
   Scaffold({
     Key key,

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

@@ -10,6 +10,7 @@ export 'theme_data.dart' show ThemeData, ThemeBrightness;
 
 const kThemeAnimationDuration = const Duration(milliseconds: 200);
 
+/// Applies a theme to descendant widgets.
 class Theme extends InheritedWidget {
   Theme({
     Key key,
@@ -20,6 +21,7 @@ class Theme extends InheritedWidget {
     assert(data != null);
   }
 
+  /// Specifies the color and typography values for descendant widgets.
   final ThemeData data;
 
   static final ThemeData _kFallbackTheme = new ThemeData.fallback();

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

@@ -27,6 +27,9 @@ const Color _kLightThemeSplashColor = const Color(0x66C8C8C8);
 const Color _kDarkThemeHighlightColor = const Color(0x40CCCCCC);
 const Color _kDarkThemeSplashColor = const Color(0x40CCCCCC);
 
+/// Holds the color and typography values for a material design theme.
+///
+/// Use this class to configure a [Theme] widget.
 class ThemeData {
 
   ThemeData.raw({