Support serialization of the compiler backbone.

pkg/compiler/lib/src/elements/common.dart

 // Copyright (c) 2015, 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.
 
 /// Mixins that implement convenience methods on [Element] subclasses.
 
 library elements.common;
 
 import '../dart2jslib.dart' show Compiler, isPrivateName;
 import '../dart_types.dart' show DartType, InterfaceType, FunctionType;
@@ skipping 108 matching lines @@
 
   @override
   bool get isPlatformLibrary => canonicalUri.scheme == 'dart';
 
   @override
   bool get isPackageLibrary => canonicalUri.scheme == 'package';
 
   @override
   bool get isInternalLibrary =>
       isPlatformLibrary && canonicalUri.path.startsWith('_');
+
+  String getLibraryOrScriptName() {
+    if (hasLibraryName()) {
+      return getLibraryName();
+    } else {
+      // Use the file name as script name.
+      String path = canonicalUri.path;
+      return path.substring(path.lastIndexOf('/') + 1);
+    }
+  }
+
+  int compareTo(LibraryElement other) {
+    if (this == other) return 0;
+    return getLibraryOrScriptName().compareTo(other.getLibraryOrScriptName());
+  }
+}
+
+abstract class CompilationUnitElementCommon implements CompilationUnitElement {
+  int compareTo(CompilationUnitElement other) {
+    if (this == other) return 0;
+    return '${script.readableUri}'.compareTo('${other.script.readableUri}');
+  }
 }
 
 abstract class ClassElementCommon implements ClassElement {
 
   @override
   Link<DartType> get allSupertypes => allSupertypesAndSelf.supertypes;
 
   @override
   int get hierarchyDepth => allSupertypesAndSelf.maxDepth;
 
@@ skipping 324 matching lines @@
       // this signature must not have more required parameters.  Having more
       // optional parameters is not a problem, they simply are never provided
       // by call sites of a call to a method with the other signature.
       int otherTotalCount = signature.parameterCount;
       return requiredParameterCount <= otherTotalCount
           && parameterCount >= otherTotalCount;
     }
     return true;
   }
 }