Add two way data binding.

sky/framework/sky-element/observe.sky

Index: sky/framework/sky-element/observe.sky
diff --git a/sky/framework/sky-element/observe.sky b/sky/framework/sky-element/observe.sky
index 23c6fb793586330c9f2faede3c0de6a8995ce6a3..415b4b8f108d50e615ff3387b5e9bd54c5aada7d 100644
--- a/sky/framework/sky-element/observe.sky
+++ b/sky/framework/sky-element/observe.sky
@@ -601,6 +601,9 @@ Observer.prototype = {
     return this.value_;
   },
 
+  setValue: function(newValue) {
+  },
+
   close: function() {
     if (this.state_ != OPENED)
       return;
@@ -887,17 +890,12 @@ CompoundObserver.prototype = createObject({
 
 function identFn(value) { return value; }
 
-function ObserverTransform(observable, getValueFn, setValueFn,
-                           dontPassThroughSet) {
+function ObserverTransform(observable, getValueFn) {
   this.callback_ = undefined;
   this.target_ = undefined;
   this.value_ = undefined;
   this.observable_ = observable;
   this.getValueFn_ = getValueFn || identFn;
-  this.setValueFn_ = setValueFn || identFn;
-  // TODO(rafaelw): This is a temporary hack. PolymerExpressions needs this
-  // at the moment because of a bug in it's dependency tracking.
-  this.dontPassThroughSet_ = dontPassThroughSet;
 }
 
 ObserverTransform.prototype = {
@@ -918,6 +916,9 @@ ObserverTransform.prototype = {
     this.callback_.call(this.target_, this.value_, oldValue);
   },
 
+  setValue: function(oldValue) {
+  },
+
   discardChanges: function() {
     this.value_ = this.getValueFn_(this.observable_.discardChanges());
     return this.value_;
@@ -927,12 +928,6 @@ ObserverTransform.prototype = {
     return this.observable_.deliver();
   },
 
-  setValue: function(value) {
-    value = this.setValueFn_(value);
-    if (!this.dontPassThroughSet_ && this.observable_.setValue)
-      return this.observable_.setValue(value);
-  },
-
   close: function() {
     if (this.observable_)
       this.observable_.close();
@@ -941,7 +936,6 @@ ObserverTransform.prototype = {
     this.observable_ = undefined;
     this.value_ = undefined;
     this.getValueFn_ = undefined;
-    this.setValueFn_ = undefined;
   }
 }