Fix ObserverTransform.getValue/setValue/discardChanges

ObserverTransform needs to conform to the contract of Observable:

-getValue/setValue do not affect the last reported value (synchronously retrieve or set the underlying value)
-discardChanges sets the last reported value to the current value

R=arv
BUG=

Review URL: https://codereview.appspot.com/41260044

src/observe.js

@@ -922,23 +922,22 @@
     },
 
     getValue: function() {
-      return this.observable_.getValue();
+      return this.getValueFn_(this.observable_.getValue());
     },
 
     discardChanges: function() {
-      return this.getValueFn_(this.observable_.discardChanges());
+      this.value_ = this.getValueFn_(this.observable_.discardChanges());
+      return this.value_;
     },
 
     deliver: function() {
       return this.observable_.deliver();
     },
 
     setValue: function(value) {
-      this.value_ = this.setValueFn_(value);
-      if (this.dontPassThroughSet_ || !this.observable_.setValue)
-        return;
-
-      return this.observable_.setValue(this.value_);
+      value = this.setValueFn_(value);
+      if (!this.dontPassThroughSet_ && this.observable_.setValue)
+        return this.observable_.setValue(value);
     },
 
     close: function() {

tests/test.js

@@ -333,8 +333,12 @@ suite('ObserverTransform', function() {
 
     observer.setValue(2);
     assert.strictEqual(obj.foo, 1);
-    assert.strictEqual(2, observer.discardChanges());
-    assertNoChanges();
+    assert.strictEqual(2, observer.getValue());
+    assertPathChanges(2, 4);
+
+    obj.foo = 10;
+    assert.strictEqual(20, observer.getValue());
+    assertPathChanges(20, 2);
 
     observer.close();
   });