Fix Dart 2 runtime errors in tests

Closes #10

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index bf2b0b0..8aa5b72 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 homepage: https://github.com/dart-lang/package_resolver
 
 environment:
-  sdk: '>=2.0.0-dev.17.0 <2.0.0'
+  sdk: '>=2.0.0-dev.37.0 <2.0.0'
 
 dependencies:
   collection: '^1.9.0'

test/current_isolate_info_test.dart

diff --git a/test/current_isolate_info_test.dart b/test/current_isolate_info_test.dart
index b554794..4891bc9 100644
--- a/test/current_isolate_info_test.dart
+++ b/test/current_isolate_info_test.dart
@@ -36,7 +36,7 @@
       var map;
       var currentIsolateMap = await PackageResolver.current.packageConfigMap;
       if (currentIsolateMap != null) {
-        map = new Map.from(currentIsolateMap);
+        map = new Map.of(currentIsolateMap);
       } else {
         // If the isolate running this test isn't using package config, create
         // one from scratch with the same resolution semantics.