Implement fromEnvironment on bool, int and String

tests/corelib/string_env_test.dart

Index: tests/corelib/string_env_test.dart
diff --git a/tests/language/closure_in_field_initializer_test.dart b/tests/corelib/string_env_test.dart
similarity index 62%
copy from tests/language/closure_in_field_initializer_test.dart
copy to tests/corelib/string_env_test.dart
index bf0198aa4b3ae78e6ad50f19e4e08a573daf80d4..4ecb1d8e185d1948827d08086f5f8e861a11dba5 100644
--- a/tests/language/closure_in_field_initializer_test.dart
+++ b/tests/corelib/string_env_test.dart
@@ -4,13 +4,8 @@
 
 import "package:expect/expect.dart";
 
-class Foo {
-  var closures = {
-    'a': (x, y) => x + y
-  };
-}
-
 main() {
-  var closures = new Foo().closures;
-  Expect.equals(6, closures['a'](4, 2));
-}
+  Expect.equals("theDefault",
+               const String.env("NOT_IN_ENV", defaultValue:"theDefault"));
+  Expect.equals("", const String.env("NOT_IN_ENV"));
+}