fix(dart/transform): Remove malfunctioning zone error handler

Remove `onError` zone callback which is consuming exceptions thrown by
the `Transformer`s and can cause `pub` to become unresponsive.

Closes #3368

modules/angular2/src/transform/template_compiler/transformer.dart

@@ -35,6 +35,6 @@ class TemplateCompiler extends Transformer {
       var transformedCode = formatter.format(await processTemplates(reader, id,
           generateChangeDetectors: options.generateChangeDetectors));
       transform.addOutput(new Asset.fromString(id, transformedCode));
-    }, errorMessage: 'Parsing ng templates failed.');
+    });
   }
 }