use proper notation for true, false, and null in doc comments

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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.
 
 library elements.modelx;
 
 import 'elements.dart';
 import '../../compiler.dart' as api;
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 280 matching lines @@
   bool isForeign(Compiler compiler) => getLibrary() == compiler.foreignLibrary;
 
   FunctionElement get targetConstructor => null;
 
   void diagnose(Element context, DiagnosticListener listener) {}
 }
 
 /**
  * Represents an unresolvable or duplicated element.
  *
- * An [ErroneousElement] is used instead of [null] to provide additional
+ * An [ErroneousElement] is used instead of [:null:] to provide additional
  * information about the error that caused the element to be unresolvable
  * or otherwise invalid.
  *
  * Accessing any field or calling any method defined on [ErroneousElement]
  * except [isErroneous] will currently throw an exception. (This might
  * change when we actually want more information on the erroneous element,
  * e.g., the name of the element we were trying to resolve.)
  *
  * Code that cannot not handle an [ErroneousElement] should use
  *   [: Element.isInvalid(element) :]
@@ skipping 2136 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }