Make it possible to construct a Stock directly

We were missing the ability to initialize one of the fields.

TBR=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1027293003

examples/stocks-fn/lib/stock_data.dart

@@ -23,7 +23,7 @@ class Stock {
   String marketCap;
   double percentChange;
 
-  Stock(this.symbol, this.name, this.lastSale, this.marketCap);
+  Stock(this.symbol, this.name, this.lastSale, this.marketCap, this.percentChange);
 
   Stock.fromFields(List<String> fields) {
     // FIXME: This class should only have static data, not lastSale, etc.