ParagraphBuilder should be able to build a paragraph

This patch start down the road of implementing text layout and painting without
the DOM. We can construct a basic paragraph consisting of a single run of text
and we can get through layout without crashing.

packages/unit/test/engine/paragraph_builder_test_disabled.dart

+import 'dart:sky';
+
+import 'package:test/test.dart';
+
+void main() {
+  test("Should be able to build and layout a paragraph", () {
+    ParagraphBuilder builder = new ParagraphBuilder();
+    builder.addText('Hello');
+    Paragraph paragraph = builder.build(new ParagraphStyle());
+    expect(paragraph, isNotNull);
+    paragraph.layout();
+  });
+}