Implement fromEnvironment on bool, int and String

tests/corelib/string_from_environment_default_value.dart

Index: tests/corelib/string_from_environment_default_value.dart
diff --git a/tests/language/call_argument_inference_test.dart b/tests/corelib/string_from_environment_default_value.dart
similarity index 62%
copy from tests/language/call_argument_inference_test.dart
copy to tests/corelib/string_from_environment_default_value.dart
index 0af5537d58ddb8eacad9f37bbbb5e0adc8295f3c..3d236e7ea4e94b984008d72f2d254fadfb05659d 100644
--- a/tests/language/call_argument_inference_test.dart
+++ b/tests/corelib/string_from_environment_default_value.dart
@@ -4,11 +4,8 @@
 
 import "package:expect/expect.dart";
 
-class A {
-  call(a) => a is num;
-}
-
 main() {
-  Expect.isTrue(new A().call(42));
-  Expect.isFalse(new A()('foo'));
+  Expect.isNull(const String.fromEnvironment('NOT_FOUND'));
+  Expect.equals('x',
+                const String.fromEnvironment('NOT_FOUND', defaultValue: 'x'));
 }