Fix a batch of DDC SDK compile errors.

sdk/lib/core/string.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * A sequence of characters.
  *
  * A string can be either single or multiline. Single line strings are
@@ skipping 125 matching lines @@
    * [defaultValue].
    *
    * Example of getting a value:
    *
    *     const String.fromEnvironment("defaultFloo", defaultValue: "no floo")
    *
    * Example of checking whether a declaration is there at all:
    *
    *     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});
 
   /**
    * Gets the character (as a single-code-unit [String]) at the given [index].
    *
    * The returned string represents exactly one UTF-16 code unit, which may be
    * half of a surrogate pair. A single member of a surrogate pair is an
    * invalid UTF-16 string:
    *
@@ skipping 633 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }