Properly pass environment variables from pub to dart2js.

sdk/lib/_internal/pub/test/transformer/dart2js/passes_along_environment_constants_test.dart

Index: sdk/lib/_internal/pub/test/transformer/dart2js/passes_along_environment_constants_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart b/sdk/lib/_internal/pub/test/transformer/dart2js/passes_along_environment_constants_test.dart
similarity index 63%
copy from sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
copy to sdk/lib/_internal/pub/test/transformer/dart2js/passes_along_environment_constants_test.dart
index 3931f5f8bd2a75436e4ba477c824393ac8488bec..89c68626c0a2f5d10e8237b73cda17e9f574a7c8 100644
--- a/sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/dart2js/passes_along_environment_constants_test.dart
@@ -12,24 +12,27 @@ import '../../serve/utils.dart';
 
 main() {
   initConfig();
-
-  integration("minify configuration overrides the mode", () {
+  integration("passes along environment constants", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
         "transformers": [{
-          "\$dart2js": {"minify": true}
+          "\$dart2js": {"environment": {'CONSTANT': 'true'}}
         }]
       }),
-      d.dir("lib", [d.dir("src", [
-        d.file("transformer.dart", REWRITE_TRANSFORMER)
-      ])])
+      d.dir("web", [
+        d.file("main.dart", """
+void main() {
+  if (const bool.fromEnvironment('CONSTANT')) {
+    print("hello");
+  }
+}
+""")
+      ])
     ]).create();
 
-    createLockFile('myapp', pkg: ['barback']);
-
     pubServe();
-    requestShouldSucceed("main.dart.js", isMinifiedDart2JSOutput);
+    requestShouldSucceed("main.dart.js", contains("hello"));
     endPubServe();
   });
 }