Clean up prints in services/*

We really shouldn't have two separate network library wrappers... I filed https://github.com/flutter/flutter/issues/2889 on that. But for now, this makes one of them have a generally better error handling story, and makes the other one only dump to the console in debug mode. It also makes both use debugPrint instead of print, and makes one use the 'mojom' prefix for mojom imports.

packages/flutter/lib/src/http/mojo_client.dart

@@ -149,8 +149,14 @@ class MojoClient {
       ByteData data = await mojo.DataPipeDrainer.drainHandle(response.body);
       Uint8List bodyBytes = new Uint8List.view(data.buffer);
       return new Response(bodyBytes: bodyBytes, statusCode: response.statusCode);
-    } catch (e) {
-      print("NetworkService unavailable $e");
+    } catch (exception) {
+      assert(() {
+        debugPrint('-- EXCEPTION CAUGHT BY NETWORKING HTTP LIBRARY -------------------------');
+        debugPrint('An exception was raised while sending bytes to the Mojo network library:');
+        debugPrint('$exception');
+        debugPrint('------------------------------------------------------------------------');
+        return true;
+      });
       return new Response(statusCode: 500);
     } finally {
       loader.close();

packages/flutter/lib/src/services/fetch.dart

@@ -4,32 +4,47 @@
 
 import 'dart:async';
 
-import 'package:mojo/mojo/url_request.mojom.dart';
-import 'package:mojo/mojo/url_response.mojom.dart';
-import 'package:mojo_services/mojo/url_loader.mojom.dart';
+import 'package:mojo/mojo/url_request.mojom.dart' as mojom;
+import 'package:mojo/mojo/url_response.mojom.dart' as mojom;
+import 'package:mojo_services/mojo/url_loader.mojom.dart' as mojom;
 
 import '../http/mojo_client.dart';
+import 'print.dart';
 
 export 'package:mojo/mojo/url_response.mojom.dart' show UrlResponse;
 
-Future<UrlResponse> fetch(UrlRequest request) async {
-  UrlLoaderProxy loader = new UrlLoaderProxy.unbound();
+Future<mojom.UrlResponse> fetch(mojom.UrlRequest request, { bool require200: false }) async {
+  mojom.UrlLoaderProxy loader = new mojom.UrlLoaderProxy.unbound();
   try {
     MojoClient.networkService.ptr.createUrlLoader(loader);
-    UrlResponse response = (await loader.ptr.start(request)).response;
+    mojom.UrlResponse response = (await loader.ptr.start(request)).response;
+    if (require200 && (response.error != null || response.statusCode != 200)) {
+      StringBuffer message = new StringBuffer();
+      message.writeln('Could not ${request.method ?? "fetch"} ${request.url ?? "resource"}');
+      if (response.error != null)
+        message.writeln('Network error: ${response.error.code} ${response.error.description ?? "<unknown network error>"}');
+      if (response.statusCode != 200)
+        message.writeln('Protocol error: ${response.statusCode} ${response.statusLine ?? "<no server message>"}');
+      if (response.url != request.url)
+        message.writeln('Final URL after redirects was: ${response.url}');
+      throw message;
+    }
     return response;
-  } catch (e) {
-    print("NetworkService unavailable $e");
-    return new UrlResponse()..statusCode = 500;
+  } catch (exception) {
+    debugPrint('-- EXCEPTION CAUGHT BY NETWORKING HTTP LIBRARY -------------------------');
+    debugPrint('An exception was raised while sending bytes to the Mojo network library:');
+    debugPrint('$exception');
+    debugPrint('------------------------------------------------------------------------');
+    return null;
   } finally {
     loader.close();
   }
 }
 
-Future<UrlResponse> fetchUrl(String relativeUrl) {
+Future<mojom.UrlResponse> fetchUrl(String relativeUrl, { bool require200: false }) {
   String url = Uri.base.resolve(relativeUrl).toString();
-  UrlRequest request = new UrlRequest()
+  mojom.UrlRequest request = new mojom.UrlRequest()
     ..url = url
     ..autoFollowRedirects = true;
-  return fetch(request);
+  return fetch(request, require200: require200);
 }

packages/flutter/lib/src/services/image_cache.dart

@@ -52,15 +52,14 @@ class _UrlFetcher implements ImageProvider {
 
   @override
   Future<ImageInfo> loadImage() async {
-    UrlResponse response = await fetchUrl(_url);
-    if (response.statusCode >= 400) {
-      print("Failed (${response.statusCode}) to load image $_url");
-      return null;
+    UrlResponse response = await fetchUrl(_url, require200: true);
+    if (response != null) {
+      return new ImageInfo(
+        image: await decodeImageFromDataPipe(response.body),
+        scale: _scale
+      );
     }
-    return new ImageInfo(
-      image: await decodeImageFromDataPipe(response.body),
-      scale: _scale
-    );
+    return null;
   }
 
   @override