Styling for text fragments

This is a completion of Eric's WIP patch:
https://codereview.chromium.org/1179663005/

Low level support for creating a paragraph that contains
runs of styled text. The styles may be nested.

The Paragraph and RenderParagraph classes have been
replaced by Inline and RenderInline. Styled text is defined
with a tree of InlineText and InlineStyle objects.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1177833012.

examples/rendering/interactive_flex.dart

@@ -75,8 +75,12 @@ porchetta bacon kevin meatball meatloaf pig beef ribs chicken. Brisket ribeye
 andouille leberkas capicola meatloaf. Chicken pig ball tip pork picanha bresaola
 alcatra. Pork pork belly alcatra, flank chuck drumstick biltong doner jowl.
 Pancetta meatball tongue tenderloin rump tail jowl boudin.""";
-  RenderParagraph paragraph = new RenderParagraph(text: meatyString, color: const Color(0xFF009900));
-  padding = new RenderPadding(padding: const EdgeDims.all(10.0), child: paragraph);
+  var text = new InlineStyle(
+      new TextStyle(color:  const Color(0xFF009900)),
+      [new InlineText(meatyString)]);
+  padding = new RenderPadding(
+      padding: const EdgeDims.all(10.0),
+      child: new RenderParagraph(text));
   column.add(padding);
 
   // Bottom cell

examples/rendering/justify_content.dart

@@ -20,7 +20,7 @@ void main() {
   var table = new RenderFlex(direction: FlexDirection.vertical);
 
   void addRow(FlexJustifyContent justify) {
-    RenderParagraph paragraph = new RenderParagraph(text: "${justify}");
+    RenderParagraph paragraph = new RenderParagraph(new InlineText("${justify}"));
     table.add(new RenderPadding(child: paragraph, padding: new EdgeDims.only(top: 20.0)));
     var row = new RenderFlex(direction: FlexDirection.horizontal);
     row.add(new RenderSolidColorBox(const Color(0xFFFFCCCC), desiredSize: new Size(80.0, 60.0)));

examples/rendering/render_paragraph.dart

@@ -31,9 +31,12 @@ andouille leberkas capicola meatloaf. Chicken pig ball tip pork picanha bresaola
 alcatra. Pork pork belly alcatra, flank chuck drumstick biltong doner jowl.
 Pancetta meatball tongue tenderloin rump tail jowl boudin.""";
 
+  var text = new InlineStyle(
+      new TextStyle(color:  const Color(0xFF009900)),
+      [new InlineText(meatyString)]);
   child = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF)),
-    child: new RenderParagraph(text: meatyString, color: const Color(0xFF009900))
+    child: new RenderParagraph(text)
   );
   flexRoot.add(child);
   child.parentData.flex = 1;

examples/rendering/touch_demo.dart

@@ -80,13 +80,14 @@ class RenderTouchDemo extends RenderBox {
 AppView app;
 
 void main() {
-  var para = new RenderParagraph(text: "Touch me!");
+  var paragraph = new RenderParagraph(new InlineText("Touch me!"));
   var stack = new RenderStack(children: [
     new RenderTouchDemo(),
-    para,
+    paragraph,
   ]);
-  // Make the paragraph not fill the whole screen so it doesn't eat events.
-  para.parentData..top = 40.0
-                 ..left = 20.0;
+  // Prevent the RenderParagraph from filling the whole screen so
+  // that it doesn't eat events.
+  paragraph.parentData..top = 40.0
+                      ..left = 20.0;
   app = new AppView(root: stack);
 }