Move image loading out of C++ into Dart

We already know how to talk to the network_service from Dart
via fetch.dart.  Might as well use that for Image loading
as well insetad of having ImageLoader do it.

As part of this I've renamed *ImageLoader to *ImageDecoder
and moved all the image loading logic into Dart.  This required
me to teach the idl system about mojo handles so that I could
pass the resulting MojoHandle from fetch.dart up through to
ImageDecoder.

R=abarth@chromium.org, jackson@google.com, hansmuller@google.com

Review URL: https://codereview.chromium.org/1173703002.

examples/stocks/lib/stock_data.dart

@@ -59,7 +59,7 @@ class StockDataFetcher {
   }
 
   void _fetchNextChunk() {
-    fetch('data/stock_data_${_currentChunk++}.json').then((Response response) {
+    fetchBody('data/stock_data_${_currentChunk++}.json').then((Response response) {
       String json = response.bodyAsString();
       JsonDecoder decoder = new JsonDecoder();
 

examples/stocks2/lib/stock_data.dart

@@ -59,7 +59,7 @@ class StockDataFetcher {
   }
 
   void _fetchNextChunk() {
-    fetch('../data/stock_data_${_currentChunk++}.json').then((Response response) {
+    fetchBody('../data/stock_data_${_currentChunk++}.json').then((Response response) {
       String json = response.bodyAsString();
       JsonDecoder decoder = new JsonDecoder();