Support configuring dart2js from the pubspec.

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

Index: sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/dart2js/compiles_generated_dart_file_test.dart b/sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
similarity index 59%
copy from sdk/lib/_internal/pub/test/transformer/dart2js/compiles_generated_dart_file_test.dart
copy to sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
index 9d73a053644c600031475df18b295104b76925ad..3931f5f8bd2a75436e4ba477c824393ac8488bec 100644
--- a/sdk/lib/_internal/pub/test/transformer/dart2js/compiles_generated_dart_file_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/dart2js/minify_configuration_overrides_mode_test.dart
@@ -12,28 +12,24 @@ import '../../serve/utils.dart';
 
 main() {
   initConfig();
-  integration("compiles a generated Dart file to JS", () {
+
+  integration("minify configuration overrides the mode", () {
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
-        "version": "0.0.1",
-        "transformers": ["myapp/transformer"]
+        "transformers": [{
+          "\$dart2js": {"minify": true}
+        }]
       }),
-      d.dir("lib", [
-        d.file("transformer.dart", dartTransformer("munge"))
-      ]),
-      d.dir("web", [
-        d.file("main.dart", """
-const TOKEN = "before";
-void main() => print(TOKEN);
-""")
-      ])
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", REWRITE_TRANSFORMER)
+      ])])
     ]).create();
 
     createLockFile('myapp', pkg: ['barback']);
 
     pubServe();
-    requestShouldSucceed("main.dart.js", contains("(before, munge)"));
+    requestShouldSucceed("main.dart.js", isMinifiedDart2JSOutput);
     endPubServe();
   });
 }