Fix a batch of DDC SDK compile errors.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index ed352e4b03534cba92a2bad0c71a78a6f59b7106..4e69f65a4a86d182ed34163a6a8b99e55bc733b7 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -143,6 +143,12 @@ abstract class String implements Comparable<String>, Pattern {
    *
    *     var isDeclared = const String.fromEnvironment("maybeDeclared") != null;
    */
+  // The .fromEnvironment() constructors are special in that we do not want
+  // users to call them using "new". We prohibit that by giving them bodies
+  // that throw, even though const constructors are not allowed to have bodies.
+  // Disable those static errors.
+  //ignore: const_constructor_with_body
+  //ignore: const_factory
   external const factory String.fromEnvironment(String name,
       {String defaultValue});