fix PathObserver to avoid try+catch

pkg/observe/test/transform_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 import 'package:observe/transform.dart';
 import 'package:unittest/compact_vm_config.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 143 matching lines @@
   }
 
   void addOutput(Asset output) {
     outs.add(output);
   }
 
   readInput(id) => throw new UnimplementedError();
   readInputAsString(id, {encoding}) => throw new UnimplementedError();
 
   static void _mockLogFn(AssetId asset, LogLevel level, String message,
-                         Span span) {
+                         span) {
     // Do nothing.
   }
 }
 
 String _sampleObservable(String annotation) => '''
 library A_foo;
 import 'package:observe/observe.dart';
 
 class A extends Observable {
   @$annotation int foo;
@@ skipping 32 matching lines @@
     "      Foo\n"
     "          get foo => __\$foo; Foo __\$foo/*D*/= 1; "
         "${_makeSetter('Foo', 'foo')} "
         "@reflectable @$meta @otherMetadata Foo get bar => __\$bar; "
         "Foo __\$bar =/*A*/2/*B*/; ${_makeSetter('Foo', 'bar')}\n"
     "          @reflectable @$meta @otherMetadata Foo get quux => __\$quux; "
         "Foo __\$quux/*C*/; ${_makeSetter('Foo', 'quux')}\n\n"
     "  @reflectable @$meta dynamic get baz => __\$baz; dynamic __\$baz; "
         "${_makeSetter('dynamic', 'baz')}\n"
     "}\n";