Cleaned up a few analyzer warnings

R=ianh@google.com

Review URL: https://codereview.chromium.org/1194583002.

sdk/lib/rendering/paragraph.dart

@@ -103,7 +103,7 @@ class TextStyle {
   }
 }
 
-class InlineBase {
+abstract class InlineBase {
   sky.Node _toDOM(sky.Document owner);
   String toString([String prefix = '']);
 }
@@ -124,7 +124,8 @@ class InlineText extends InlineBase {
 
 class InlineStyle extends InlineBase {
   InlineStyle(this.style, this.children) {
-    assert(style != null && children != null);
+    assert(style != null);
+    assert(children != null);
   }
 
   final TextStyle style;
@@ -182,7 +183,7 @@ class RenderParagraph extends RenderBox {
     markNeedsLayout();
   }
 
-  sky.Element _layout(BoxConstraints constraints) {
+  void _layout(BoxConstraints constraints) {
     _layoutRoot.maxWidth = constraints.maxWidth;
     _layoutRoot.minWidth = constraints.minWidth;
     _layoutRoot.minHeight = constraints.minHeight;

sdk/lib/widgets/basic.dart

@@ -419,7 +419,7 @@ class WidgetToRenderBoxAdapter extends RenderObjectWrapper {
     super.syncRenderObject(old);
     if (old != null) {
       assert(old is WidgetToRenderBoxAdapter);
-      assert(root == old.renderBox);
+      assert(root == old.root);
     }
   }