fix observe package to be strong mode clean

test/transformer_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.
 @TestOn('vm')
 import 'dart:async';
 import 'package:barback/barback.dart';
 import 'package:observe/transformer.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:test/test.dart';
 
@@ skipping 108 matching lines @@
       var begin = output.indexOf(constructor) + constructor.length - 1;
       var end = output.indexOf(';', begin);
       if (end == -1) end = output.length;
       var init = output.substring(begin, end).trim().replaceAll('  ', ' ');
       expect(init, expected);
     });
   });
 }
 
 /// Helper that applies the transform by creating mock assets.
-Future<String> _transform(String code) {
-  return Chain.capture(() {
+Future _transform(String code) {
+  return Chain.capture(() async {
     var id = new AssetId('foo', 'a/b/c.dart');
     var asset = new Asset.fromString(id, code);
     var transformer = new ObservableTransformer();
-    return transformer.isPrimary(asset).then((isPrimary) {
-      expect(isPrimary, isTrue);
-      var transform = new _MockTransform(asset);
-      return transformer.apply(transform).then((_) {
-        expect(transform.outs, hasLength(2));
-        expect(transform.outs[0].id, id);
-        expect(transform.outs[1].id, id.addExtension('._buildLogs.1'));
-        return transform.outs.first.readAsString();
-      });
-    });
+    bool isPrimary = await transformer.isPrimary(asset);
+    expect(isPrimary, isTrue);
+    var transform = new _MockTransform(asset);
+    await transformer.apply(transform);
+
+    expect(transform.outs, hasLength(2));
+    expect(transform.outs[0].id, id);
+    expect(transform.outs[1].id, id.addExtension('._buildLogs.1'));
+    return transform.outs.first.readAsString();
   });
 }
 
 class _MockTransform implements Transform {
   bool shouldConsumePrimary = false;
   List<Asset> outs = [];
   Asset _asset;
   TransformLogger logger = new TransformLogger(_mockLogFn);
   Asset get primaryInput => _asset;
 
@@ skipping 64 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";