Dart2js option to dump info about compilation

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 384ca4e161031bab157385ace6a22772f855362e..d3809cec01a08ee15a081dc6dc916136a3bdc57e 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1945,6 +1945,12 @@ class JavaScriptBackend extends Backend {
           element, "@NoSideEffects() should always be combined with @NoInline");
     }
   }
+
+  CodeBuffer codeOf(Element element) {
+    return generatedCode.containsKey(element)
+        ? jsAst.prettyPrint(generatedCode[element], compiler)
+        : null;
+  }
 }
 
 /// Records that [constant] is used by [user.element].