Introduce a LeafRenderObjectWrapper abstract class so that...

Introduce a LeafRenderObjectWrapper abstract class so that RenderObjectWrappers that are leaves don't have to all redundantly assert that they're not misused.

Also, make images use their URL as their src.

Other misc code cleanup.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1191443009.

sdk/lib/widgets/basic.dart

@@ -365,7 +365,7 @@ class Flexible extends ParentDataNode {
     : super(child, new FlexBoxParentData()..flex = flex, key: key);
 }
 
-class Inline extends RenderObjectWrapper {
+class Inline extends LeafRenderObjectWrapper {
   Inline({ String key, this.text }) : super(key: key);
 
   RenderParagraph get root => super.root;
@@ -378,15 +378,10 @@ class Inline extends RenderObjectWrapper {
     root.inline = text;
   }
 
-  void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
-    assert(false);
-    // Inline does not support having children currently
-  }
-
 }
 
 class Text extends Component {
-  Text(data, { String key, TextStyle this.style }) : data = data, super(key: key);
+  Text(data, { String key, TextStyle this.style }) : super(key: key);
   final String data;
   final TextStyle style;
   bool get interchangeable => true;
@@ -397,13 +392,15 @@ class Text extends Component {
   }
 }
 
-class Image extends RenderObjectWrapper {
+class Image extends LeafRenderObjectWrapper {
 
   Image({
-    String key,
-    this.src,
+    src,
     this.size
-  }) : super(key: key);
+  }) : src = src,
+       super(key: src) {
+    assert(src != null);
+  }
 
   RenderImage get root => super.root;
   RenderImage createNode() => new RenderImage(this.src, this.size);
@@ -417,17 +414,12 @@ class Image extends RenderObjectWrapper {
     root.requestedSize = size;
   }
 
-  void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
-    assert(false);
-    // Image does not support having children currently
-  }
-
 }
 
-class WidgetToRenderBoxAdapter extends RenderObjectWrapper {
+class WidgetToRenderBoxAdapter extends LeafRenderObjectWrapper {
 
   WidgetToRenderBoxAdapter(RenderBox renderBox)
-    : this.renderBox = renderBox,
+    : renderBox = renderBox,
       super(key: renderBox.hashCode.toString());
 
   RenderBox get root => super.root;
@@ -443,12 +435,6 @@ class WidgetToRenderBoxAdapter extends RenderObjectWrapper {
     }
   }
 
-  void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
-    assert(false);
-    // WidgetToRenderBoxAdapter cannot have Widget children; by
-    // definition, it is the transition out of the Widget world.
-  }
-
   void remove() {
     parent.detachChildRoot(this);
     super.remove();

sdk/lib/widgets/widget.dart

@@ -549,6 +549,20 @@ abstract class RenderObjectWrapper extends Widget {
 
 }
 
+abstract class LeafRenderObjectWrapper extends RenderObjectWrapper {
+
+  LeafRenderObjectWrapper({ String key }) : super(key: key);
+
+  void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+  }
+
+  void detachChildRoot(RenderObjectWrapper child) {
+    assert(false);
+  }
+
+}
+
 abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
 
   OneChildRenderObjectWrapper({ String key, Widget child })