Cleaned up a few analyzer warnings

sky/sdk/lib/rendering/paragraph.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import 'box.dart';
 import 'object.dart';
 
 enum FontWeight {
@@ skipping 85 matching lines @@
     if (fontWeight != null)
       result.add('${prefix}fontWeight: $fontWeight');
     if (textAlign != null)
       result.add('${prefix}textAlign: $textAlign');
     if (result.isEmpty)
       return '${prefix}<no style specified>';
     return result.join('\n');
   }
 }
 
-class InlineBase {
+abstract class InlineBase {
   sky.Node _toDOM(sky.Document owner);
   String toString([String prefix = '']);
 }
 
 class InlineText extends InlineBase {
   InlineText(this.text) {
     assert(text != null);
   }
 
   final String text;
 
   sky.Node _toDOM(sky.Document owner) {
     return owner.createText(text);
   }
 
   String toString([String prefix = '']) => '${prefix}InlineText: "${text}"';
 }
 
 class InlineStyle extends InlineBase {
   InlineStyle(this.style, this.children) {
-    assert(style != null && children != null);
+    assert(style != null);
+    assert(children != null);
   }
 
   final TextStyle style;
   final List<InlineBase> children;
 
   sky.Node _toDOM(sky.Document owner) {
     sky.Element parent = owner.createElement('t');
     style._applyToCSSStyle(parent.style);
     for (InlineBase child in children) {
       parent.appendChild(child._toDOM(owner));
@@ skipping 37 matching lines @@
   InlineBase _inline;
   BoxConstraints _constraintsForCurrentLayout;
 
   InlineBase get inline => _inline;
   void set inline (InlineBase value) {
     _inline = value;
     _layoutRoot.rootElement.setChild(_inline._toDOM(_document));
     markNeedsLayout();
   }
 
-  sky.Element _layout(BoxConstraints constraints) {
+  void _layout(BoxConstraints constraints) {
     _layoutRoot.maxWidth = constraints.maxWidth;
     _layoutRoot.minWidth = constraints.minWidth;
     _layoutRoot.minHeight = constraints.minHeight;
     _layoutRoot.maxHeight = constraints.maxHeight;
     _layoutRoot.layout();
     _constraintsForCurrentLayout = constraints;
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     _layout(constraints);
@@ skipping 44 matching lines @@
   }
 
   // we should probably expose a way to do precise (inter-glpyh) hit testing
 
   String debugDescribeSettings(String prefix) {
     String result = '${super.debugDescribeSettings(prefix)}';
     result += '${prefix}inline: ${inline}\n';
     return result;
   }
 }