fix #136, support watch exprs and two way bindings for component fields

test/data/expected/custom_radio_button_test.html.txt

Index: test/data/expected/custom_radio_button_test.html.txt
diff --git a/test/data/expected/radio_button_test.html.txt b/test/data/expected/custom_radio_button_test.html.txt
similarity index 72%
copy from test/data/expected/radio_button_test.html.txt
copy to test/data/expected/custom_radio_button_test.html.txt
index a9164441d3561c96dcacc882752019ab4618b93a..3ec2f2298eb6ed3d1d879f8cd695f061f0a4d8c1 100644
--- a/test/data/expected/radio_button_test.html.txt
+++ b/test/data/expected/custom_radio_button_test.html.txt
@@ -8,11 +8,13 @@ layer at (0,0) size 800x71
     RenderBody {BODY} at (8,8) size 784x50
       RenderBlock {FORM} at (0,0) size 784x21
         RenderBlock {INPUT} at (5,3) size 13x13
-        RenderText {#text} at (21,1) size 34x19
-          text run at (21,1) width 34: "Foo! "
-        RenderBlock {INPUT} at (60,3) size 13x13
-        RenderText {#text} at (76,1) size 27x19
-          text run at (76,1) width 27: "Bar!"
+        RenderText {#text} at (21,1) size 38x19
+          text run at (21,1) width 4: " "
+          text run at (25,1) width 34: "Foo! "
+        RenderBlock {INPUT} at (64,3) size 13x13
+        RenderText {#text} at (80,1) size 31x19
+          text run at (80,1) width 4: " "
+          text run at (84,1) width 27: "Bar!"
       RenderBlock {PRE} at (0,34) size 784x16
         RenderText {#text} at (0,0) size 88x16
           text run at (0,0) width 88: "You picked "